Potem muszę wypełnić także drugi wniosek dla mojej siostry.

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Breakdown of Potem muszę wypełnić także drugi wniosek dla mojej siostry.

• mój: my
• musieć: to have to
• dla: for

Questions & Answers about Potem muszę wypełnić także drugi wniosek dla mojej siostry.

Why is it muszę, and what verb is that from?

Muszę is the 1st person singular form of the verb musieć (to have to / must).

So:

• muszę = I have to / I must
• musisz = you have to
• musi = he/she/it has to

In this sentence, muszę matches the implied subject I.

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Why is the verb wypełnić used here instead of wypełniać?

This is about aspect, which is very important in Polish.

• wypełniać = imperfective: the process of filling out, doing it in general, repeatedly, or without focusing on completion
• wypełnić = perfective: to fill out completely, with the result in mind

After muszę, Polish can use either aspect depending on meaning:

• Muszę wypełniać formularze codziennie. = I have to fill out forms every day.
• Muszę wypełnić ten formularz. = I have to fill out this form (and finish it).

In your sentence, the idea is completing a specific application, so wypełnić is natural.

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What exactly does także mean here? Is it the same as też?