Если ты ошибся в бланке, возьми другой у женщины на почте.

Russian often uses the past tense in an if clause when the action is viewed as already completed before the result happens.

So:

Если ты ошибся в бланке, возьми другой...
means something like: If you have made a mistake on the form, take another one...

The idea is:

1. first, the mistake happened;
2. then, as a result, you should take another form.

A present-tense verb would not sound natural here. Russian prefers the completed-action idea.

Ошибся is the masculine singular past-tense form of ошибиться, which means to make a mistake or to be mistaken.

Breakdown:

• ошибиться = infinitive
• ошибся = past tense, masculine
• ошиблась = past tense, feminine
• ошиблись = past tense, plural

So if the speaker were talking to a woman, it would be:

Если ты ошиблась в бланке...

In this verb, -ся is just part of the normal dictionary form: ошибиться / ошибаться.

For learners, the important point is that Russian usually says:

• ошибиться = to make a mistake
• ошибаться = to be making mistakes / to make mistakes regularly

So here -ся does not need to be translated literally as oneself. It is simply how this verb is formed.

You should learn it as a whole verb:

• Я ошибся. = I made a mistake.
• Ты ошибся. = You made a mistake.

В бланке uses the prepositional case after в.

• бланк = form, blank
• в бланке = in/on the form

After ошибиться, Russian commonly uses в + prepositional to say where the mistake was made:

• ошибиться в имени = make a mistake in the name
• ошибиться в адресе = make a mistake in the address
• ошибиться в бланке = make a mistake on/in the form

English often says on the form, but Russian says in the form with в.

Возьми is the imperative of взять, a perfective verb.

Russian often uses:

• perfective imperative for one complete action
• imperfective imperative for a process, repeated action, or more general instruction

Here the speaker means a single completed action:

Take another one.

So возьми is the natural choice.

Compare:

• возьми другой = take another one
• бери другой = take another one / go ahead and take another one, with a slightly different feel

In this sentence, возьми is the most standard and neutral.

Russian often leaves out a noun when it is obvious from context.

So:

возьми другой
literally = take another

but it clearly means:

take another form

The omitted noun is бланк.

This is very natural in Russian, just like in English:

• I need a pen. Do you have another?
• Возьми другой. = Take another one.

It is accusative singular masculine, agreeing with the implied noun бланк.

Why does it look like nominative? Because бланк is masculine inanimate, and for masculine inanimate nouns/adjectives, the accusative looks the same as the nominative.

So:

• nominative: другой бланк
• accusative: возьми другой бланк

Since бланк is omitted, only другой remains.

Yes, but у + genitive also often means from a person, especially when receiving or taking something from them.

So:

возьми другой у женщины
means
take another one from the woman

This is very natural Russian when the source is a person.

Examples:

• спроси у преподавателя = ask the teacher
• возьми у него ручку = take a pen from him
• купи у женщины газету = buy a newspaper from the woman

Here женщины is genitive singular of женщина.

На почте means at the post office.

Russian often uses на with places understood as institutions, workplaces, or service locations:

• на почте = at the post office
• на работе = at work
• на вокзале = at the station

So у женщины на почте means:

from the woman at the post office

It does not necessarily focus on being physically inside the building; it refers more naturally to the location/service point.

Yes. Russian word order is flexible.

This sentence could be rephrased in other ways, for example:

• Если ты ошибся в бланке, возьми другой бланк у женщины на почте.
• Если ты ошибся в бланке, возьми у женщины на почте другой.

The original version is natural because it keeps the instruction short and places другой early, which highlights the important action: take another one.

So the original word order is not random, but it is not the only possible order.

Not strictly. Russian often drops subject pronouns when the meaning is clear from the verb form.

So you could say:

Если ошибся в бланке, возьми другой у женщины на почте.

That would still be understandable.

However, ты is included here because it makes the sentence more direct and conversational, as if someone is speaking personally to one person.

So:

• with ты = a bit more explicit, direct
• without ты = slightly more concise

Both are possible.