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To align two subtitle files sentence by sentence, first work out whether the mismatch is in timing, in how each language splits its text into cues, or in both. Correct the timing where it is off, match complete sentences across the two languages, and only then merge the files. An automatic merge that pairs cues by time overlap is a convenient starting draft. It does not confirm that two paired cues say the same thing.
Start by identifying what is out of step
Two subtitle files can differ in three ways, and each needs a different fix. Sort your problem into one of these before you edit anything:
- A constant offset. Every cue in one file appears a fixed amount of time early or late. The first, middle, and last cues all show the same gap against the video.
- A drifting offset. The gap is small at the start and grows toward the end, or it jumps at certain scenes. This usually happens when one file was made from a different frame rate, a different cut of the video, or a different transcription pass.
- Different segmentation. Timing may be correct, but one language splits a sentence across two or three cues while the other keeps it in one. Translation also changes word order and sentence length, so cue N in one file rarely lines up with cue N in the other.
Most real pairs show a combination. Check timing first, because it is easier to judge whether a sentence matches once both tracks appear at the right moment.
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Compare the opening, a middle section, and the closing cues against the same playback position in the video. Note the gap at each point. If the gap is the same everywhere, apply a single global shift. If it changes, you need a correction that works on individual points or segments.
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Subtitle Edit’s official documentation lists global time adjustment, point synchronization, and point synchronization via another subtitle among its timing features, and it supports SRT, WebVTT, and ASSA files. Point sync via another subtitle is useful when a second track already has correct timing: you match a few reference cues to the correct track and let the program adjust the rest.
Subaligner’s version 0.3.4 documentation, dated 2023-08-25, describes two approaches. A single-stage alignment shifts all segments by the same amount. A dual-stage alignment shifts each segment individually and can stretch it. Its listed formats include SubRip, TTML, WebVTT, ASS, MicroDVD, MPL2, EBU STL, and SAMI. Because software changes, confirm that the version you have installed behaves the way its documentation describes.
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| Situation | Approach | What it changes | Limitation |
|---|---|---|---|
| Constant offset across the file | Global shift (Subtitle Edit global time adjustment; Subaligner single-stage) | Moves every cue by the same amount | Cannot fix drift or a cut that changed timing partway through |
| Offset changes at a few known points | Point sync, or point sync via another subtitle | Adjusts timing around reference cues you choose | Quality depends on choosing correct reference cues |
| Offset varies by segment | Segment-level alignment (Subaligner dual-stage, with optional stretching) | Shifts and can stretch each segment individually | Stretching can distort timing if segment boundaries are wrong |
Match sentences, not cue numbers
Cue numbers are a counter, not a meaning. Before you merge anything, read each file as text and decide which cues belong to the same sentence. Where one language splits a sentence across cues, join those cues in a working view, then match the complete thought to the complete thought in the other language. Expect word order to differ. A sentence that ends in the middle of a clause in one language may end in the middle of a different clause in the other.
If one language has many short cues and the other has few long ones, you have two options:
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- Keep the original cues. This avoids inventing new timings. The bilingual pairs will be less exact, but the timing you see is the timing the creator delivered.
- Rebuild full-sentence cues. This improves correspondence, but the new start and end times must be estimated or checked by hand.
A 2022 technical report from the EASIER Consortium describes one way to rebuild sentences. It extracts whole sentences from subtitle cues, estimates each sentence’s start and end time from word counts, and then rebuilds subtitle files. The report also notes failures when a sentence contains several punctuation marks. Treat word-count timing as a starting estimate. Check it against the audio or video, and correct it where it drifts.
Merge the files in Subtitle Edit
Subtitle Edit’s official documentation describes a merge tool that combines two independent subtitle files into one bilingual track. Use it after the timing and sentence matching are reasonable, not before.
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- Make working copies of both source files. Keep the originals untouched.
- In Subtitle Edit, open Tools → Merge two subtitles.
- Load the first file on one side and the second on the other side. Swap them if the controlling-language file should appear on the top line.
- Select an output format. The documentation lists SRT and ASS, and the choice affects layout (see the next section).
- Review the live preview. Scroll through the opening, middle, and end, and check that the pairs make sense as sentences.
- Merge, then open the result and correct any pairs that were matched by overlap but belong to different sentences.
The merge pairs cues that overlap in time, even when their start and end times do not match exactly. That is useful when both tracks are already timed well, but it can pair a fragment with the wrong sentence when cue boundaries differ. The documentation describes overlap pairing and does not claim that it guarantees sentence equivalence, so the pairing review in step 6 is necessary.
Choose the output format for your playback target
The two output formats present the bilingual text in different ways.
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| Output | How the two languages appear | Styling and placement | Check before you rely on it |
|---|---|---|---|
| SRT | For each overlapping pair, subtitle 1 text sits above subtitle 2 text in the same cue | Stacked text, with no separate styling for each language | Confirm that your player shows two lines cleanly at the size you use |
| ASS | Each language can use its own style | Separate styles and top or bottom alignment for each language | Confirm that the player supports ASS styling; behavior varies across devices and apps |
Subtitle Edit’s documentation describes these outputs. It does not prove that either format will display the same way on every device, so preview the final file in the player you intend to use.
Check the result before you export
- Original files are preserved and unchanged.
- No cue from either language has been dropped or duplicated during the merge.
- Sentence order matches in both languages, from the first cue to the last.
- Time coverage is complete: no stretch of the video has blank bilingual text where both languages should appear.
- Each paired sentence appears on screen long enough to read in both languages.
- The file plays correctly in the intended player at the points you checked earlier, including the opening, a middle section, and the end.
Troubleshooting common problems
- Pairs look right at the start but drift later. The offset is not constant. Return to the timing step and use point sync or segment-level alignment rather than a global shift.
- One language’s sentence appears above the other language’s previous sentence. The cues are paired by overlap, but the boundaries differ. Join the fragments in the working view and repair the pairs manually.
- Text from one language is missing. A cue was not paired or was merged into a neighboring cue. Compare the cue counts in the merged file with the originals.
- Stacked text overlaps the picture. Try a different output format or adjust the player’s subtitle position, and check the final file in the target player.
- Rebuilt sentences start or end at the wrong moment. Word-count timing is an estimate. Review each rebuilt cue against the audio and correct its start or end time.
Subtitle Edit’s documentation, Subaligner’s version 0.3.4 documentation, and the EASIER Consortium’s 2022 report all describe these operations, but none of them shows a tested end-to-end workflow for your specific pair of files. Work through a short clip first, then apply the same checks to the full file.
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