Мало того что в сочинении была опечатка, так ещё я пропустил целую строку.

It is a fixed emphatic pattern used to add one negative fact on top of another.

The idea is:

• one bad thing was already true
• and on top of that, another bad thing happened

So the structure means something like:

• not only ... , but on top of that ...
• as if that weren’t enough ...
• to make matters worse ...

Here, the speaker is complaining that there was already a typo in the essay, and then an additional mistake happened: they skipped a whole line.

Here так ещё is not meant literally word by word. In this construction, it works as an emphatic connector meaning roughly and what’s more or and on top of that.

So in this sentence:

• мало того что ... = not only that...
• так ещё ... = but moreover / on top of that...

The так strengthens the second part and makes the whole phrase sound more idiomatic and expressive.

You could compare:

• Мало того что..., так ещё... — very natural, emphatic
• Мало того что..., ещё... — possible, but less idiomatic here

Сочинении is in the prepositional case.

The preposition в often takes the prepositional case when it means in or inside in a non-motion sense. Here it means in the essay.

• сочинение → base form
• в сочинении → in the essay

So:

• в сочинении была опечатка = there was a typo in the essay

Because this is an existential-type sentence: it introduces the fact that something existed or was present.

• была опечатка = there was a typo

In Russian, the noun is normally in the nominative in this kind of sentence.

• опечатка = nominative
• опечаткой = instrumental

If you used опечаткой, it would suggest a different structure, more like it was a typo, not there was a typo.

So:

• В сочинении была опечатка = There was a typo in the essay.

Because it agrees with опечатка, which is a feminine singular noun.

In the past tense, Russian verbs agree in gender and number:

• masculine: был
• feminine: была
• neuter: было
• plural: были

Since опечатка is feminine, you get:

• была опечатка

Опечатка is more specific than ошибка.

• ошибка = mistake, error in general
• опечатка = typo, misprint, accidental writing/typing error

So опечатка usually means the kind of error where the wrong letter, word, or form appears by accident.

In this sentence, it is not just saying there was some error in the essay, but specifically a typo.

Here пропустил means skipped, left out, or omitted.

So:

• я пропустил целую строку = I skipped / left out a whole line

This is a very common use of пропустить with text, speech, reading, copying, and writing. It often means that something should have been included, but accidentally wasn’t.

Examples of the same idea:

• пропустить слово = to skip a word
• пропустить абзац = to skip a paragraph
• пропустить ошибку can mean to overlook an error

Пропустил is the past tense of the perfective verb пропустить.

The perfective is used because the sentence refers to a completed result:

• the line ended up being omitted

This is a one-time finished mistake, so perfective fits naturally.

If you used the imperfective пропускал, it would sound more like:

• repeated action
• habitual behavior
• or focus on the process rather than the completed result

Here the speaker is talking about a specific finished error, so пропустил is the normal choice.

Because строку is the direct object of пропустил, so it is in the accusative case.

• строка = nominative
• строку = accusative singular

The adjective целую agrees with строку:

• целая строка = a whole line
• целую строку = a whole line (as object)

Here целую adds emphasis. It means not just a line, but an entire line.

So the phrase suggests: this was not a tiny omission — a whole line was skipped.

The overall pattern is fairly stable, but there is some flexibility.

The most natural order here is:

• Мало того что ..., так ещё ...

That said, Russian can move things around for emphasis. For example:

• Мало того что в сочинении была опечатка, так я ещё пропустил целую строку.

This version puts more emphasis on я.

You may also hear related variants such as:

• Мало того что..., да ещё...
• Мало того что..., ещё и...

But так ещё is very natural in expressive speech and writing.

You will often see this written as:

• Мало того, что ..., так ещё ...

That punctuation is very common and widely accepted.

You may also come across мало того что written without the internal comma, especially in less formal or less carefully edited text. But for a learner, the safest model is:

• Мало того, что в сочинении была опечатка, так ещё я пропустил целую строку.

In any case, there is definitely a comma before the second part:

• ..., так ещё ...

because that separates the two clauses.