Det var først da min chef sendte rettelsen tilbage, at jeg så den lille tastfejl i overskriften.

This is an emphatic Danish structure, similar to English It was only when ... that ....

So:

• Det var først da min chef sendte rettelsen tilbage, at jeg så ...
• literally: It was only when my boss sent the correction back that I saw ...

It highlights the timing very strongly. The speaker is not just saying when they saw the typo, but emphasizing that they did not notice it earlier.

A more neutral version would be:

• Jeg så først den lille tastfejl i overskriften, da min chef sendte rettelsen tilbage.

That means roughly the same thing, but the original sentence is more focused and dramatic.

It often does mean first, but in this kind of sentence it usually means only, not until, or only then.

So here:

• Det var først da ... at ...
• means It was only when ... that ...

This is a very common use of først in Danish.

Compare:

• Jeg spiste først dessert. = I ate dessert first.
• Jeg forstod det først senere. = I only understood it later.

So the exact meaning depends on context.

Because this sentence refers to a specific event in the past.

• da = when, for a particular past occasion
• når = when, for repeated events, general truths, or future meaning

Here the speaker is talking about one particular moment:

• da min chef sendte rettelsen tilbage
  = when my boss sent the correction back

If you used når, it would sound more like a general habit or repeated situation.

In this structure, at introduces the clause that contains the main point:

• Det var først da ..., at jeg så ...

This is the standard pattern in Danish cleft-like sentences:

• Det var X, at Y
• It was X that Y

So:

• Det var først da min chef sendte rettelsen tilbage, at jeg så den lille tastfejl ...

The at corresponds to English that in It was only when ... that I saw ...

In everyday speech, some speakers may sometimes leave out at, but in careful written Danish it is very normal here.

In Danish, chef usually means boss, manager, or head of something.

So:

• min chef = my boss

This is a false friend for English speakers, because English chef means a professional cook. Danish uses kok for cook/chef in that sense.

Examples:

• Min chef er på møde. = My boss is in a meeting.
• Han er kok på en restaurant. = He is a cook at a restaurant.

Rettelsen means the correction.

It comes from rettelse, which means:

• a correction
• an amendment
• something that has been corrected

In this context, it probably means a corrected version of a text or document that the boss sent back.

The ending -en makes it definite:

• en rettelse = a correction
• rettelsen = the correction

Because in Danish, the object often comes before short adverbs or particles like tilbage.

So the natural order is:

• sendte rettelsen tilbage
  = sent the correction back

Here:

• sendte = sent
• rettelsen = the correction
• tilbage = back

This word order is very normal in Danish.

You may also notice that Danish often places particles like tilbage, ud, op, ind after the object, especially when the object is a noun phrase.

Good question: så can be different words.

In this sentence, så is the past tense of at se (to see):

• at se = to see
• ser = see/sees
• så = saw
• set = seen

So:

• jeg så den lille tastfejl = I saw the small typo

There is also another common så, meaning then, so, or thus. You tell them apart from context.

Here, after jeg, a verb is expected, so så must be saw.

Because when a noun is definite and has an adjective before it, Danish usually uses:

• den/det/de + adjective + noun in base form

So:

• en lille tastfejl = a small typo
• den lille tastfejl = the small typo

You do not normally say den lille tastfejlen.

This is a very important Danish pattern:

• bilen = the car
• den røde bil = the red car

Notice how the noun loses the definite ending when den + adjective is used.

Tastfejl means typing mistake or typo.

It is a compound noun:

• tast = key / keystroke
• fejl = mistake / error

So literally it is something like keying error, but in natural English it is usually typo.

Danish uses compound nouns very often, so this is a useful example of how two words combine into one.

Because the speaker means a specific headline/title: the headline.

• en overskrift = a headline / a heading / a title
• overskriften = the headline / the heading / the title

So:

• i overskriften = in the headline

Unlike den lille tastfejl, there is no adjective before overskrift, so Danish just uses the normal definite ending -en on the noun.

It can mean several related things, depending on context:

• headline
• heading
• title

In this sentence, if the text is an article or document, overskriften could be translated as:

• the headline
• the title
• the heading

The exact English word depends on what kind of text it is.

Danish does put the finite verb second in main clauses, but subordinate clauses follow different word order.

Here you have two subordinate clauses inside a larger emphatic structure:

• da min chef sendte rettelsen tilbage
• at jeg så den lille tastfejl i overskriften

After subordinators like da and at, Danish normally uses subject + verb order:

• min chef sendte
• jeg så

So this is completely normal.

The verb-second rule applies to main clauses like:

• Min chef sendte rettelsen tilbage.
• Jeg så den lille tastfejl.

But after da or at, you do not switch to verb-second in the same way.

Yes. A simpler, less emphatic version would be:

• Jeg så først den lille tastfejl i overskriften, da min chef sendte rettelsen tilbage.

That still means:

• I only noticed the small typo in the headline when my boss sent the correction back.

The original version:

• Det var først da ..., at jeg så ...

puts stronger focus on the timing. It sounds a bit more deliberate and polished in writing.