Учитель попросил поставить запятую перед «потому что».

Because it is past tense, masculine, singular.

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

• учитель попросил = the teacher asked
• учительница попросила = the female teacher asked
• учителя попросили = the teachers asked

Since учитель is masculine singular, the verb is попросил.

Russian often omits subject pronouns when they are obvious from the verb form or from context.

So instead of saying:

• Он попросил поставить запятую...

Russian very naturally says:

• Учитель попросил поставить запятую...

The noun учитель already tells you who did the action, so он is unnecessary.

Учитель is the subject of the sentence, in the nominative singular.

It is the person who performed the action of asking.

So the structure is basically:

• Учитель = subject
• попросил = verb
• поставить запятую... = what he asked someone to do

This is a very common Russian pattern:

• попросить + infinitive

It means to ask someone to do something.

Examples:

• Мама попросила закрыть окно. = Mom asked someone to close the window.
• Учитель попросил поставить запятую. = The teacher asked someone to put a comma.

The person being asked can be stated:

• Учитель попросил ученика поставить запятую.

But it can also be left out if it is obvious or unimportant.

This is about aspect.

• поставить = perfective
• ставить = imperfective

Here the teacher asked for one completed action: to put the comma in the correct place. That is why Russian uses the perfective verb поставить.

Compare:

• поставить запятую = to put in the comma
• ставить запятые = to be putting commas / to put commas regularly or as a process

So поставить fits best because the sentence refers to a single finished result.

Because запятую is the accusative singular form.

The noun запятая is feminine, and here it is the direct object of поставить:

• nominative: запятая = a comma
• accusative: запятую = a comma, as the thing being put

So:

• поставить что? → запятую

This is a standard feminine noun pattern:

• книга → книгу
• машина → машину
• запятая → запятую

Yes, it is a very normal and idiomatic Russian expression.

Russian commonly uses ставить / поставить with punctuation marks:

• поставить запятую = put a comma
• поставить точку = put a period
• поставить вопросительный знак = put a question mark

So even though English might also say insert or add a comma, Russian very naturally says put a comma.

Перед means before or in front of.

In this sentence, it is used in the punctuation sense:

• перед потому что = before because

So the phrase:

• поставить запятую перед ...

means:

• to put a comma before ...

This is a very common pattern when talking about spelling or punctuation rules.

Yes, перед normally takes the instrumental case:

• перед домом
• перед уроком
• перед словом

But here потому что is not functioning like an ordinary declined noun. It is being mentioned as a quoted expression — basically, the sentence is talking about the words themselves.

Quoted words and fixed expressions are often treated as indeclinable citations, so they usually stay in their dictionary form:

• перед «потому что»
• после «если»
• без слова «нет»

So the preposition still has its usual meaning, but the quoted expression does not change form.

Because the sentence is mentioning the words themselves, not using them normally inside the sentence.

Compare:

• Я ушёл, потому что устал.
  Here потому что is actually functioning as a conjunction: because.

• Учитель попросил поставить запятую перед «потому что».
  Here потому что is being talked about as a piece of language.

Russian often uses «...» for this. These are the standard Russian quotation marks, sometimes called angle quotes.

In English, you might use quotation marks or italics. Russian usually uses «...» in normal printed text.

Yes, Russian word order is fairly flexible, but the version you have is the most neutral and natural.

• Учитель попросил поставить запятую перед «потому что».

This order is clear and standard:

1. who did it
2. what they did
3. what action they requested
4. where the comma should go

You can change word order for emphasis, but it may sound less neutral. For example:

• Перед «потому что» учитель попросил поставить запятую.

This is possible, but it emphasizes where the comma should go.

Yes. In ordinary Russian sentences, потому что usually introduces a subordinate clause, and there is normally a comma before it:

• Я ушёл, потому что устал.

So the teacher’s instruction refers to a very common punctuation rule.

However, advanced learners should know that punctuation with потому что can sometimes be more nuanced. In certain structures, the conjunction may be split, and the comma placement can change depending on meaning and emphasis.

But as a basic rule, put a comma before потому что is a very good default.

Yes.

Russian can leave out the person being asked if it is understood from context. So this sentence can mean:

• The teacher asked the student
• The teacher asked the class
• The teacher asked someone present

If Russian wanted to name that person, it could:

• Учитель попросил ученика поставить запятую перед «потому что».

So the original sentence is natural because the omitted person is either obvious or unimportant.