Muszę uzasadnić odpowiedź.

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

So:

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

In Polish, the subject pronoun ja (I) is usually omitted, because the verb ending already shows who the subject is. So Muszę by itself already means I must / I have to.

Polish often leaves out subject pronouns when they are clear from the verb form.

• Muszę uzasadnić odpowiedź.
• literally: Must-I justify the answer/answer

Because muszę clearly means I must, adding ja is usually unnecessary.

You can say Ja muszę uzasadnić odpowiedź, but that sounds more emphatic, like:

• I have to justify the answer
• It’s me who has to justify it

So the version without ja is the most neutral and natural.

After musieć (to have to / must), Polish normally uses an infinitive.

Pattern:

• musieć + infinitive

Examples:

• Muszę iść. = I have to go.
• Muszę pracować. = I have to work.
• Muszę uzasadnić odpowiedź. = I have to justify the answer.

So uzasadnić is in the infinitive because it is the action that the speaker must do.

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

• uzasadniać = imperfective
• uzasadnić = perfective

Very roughly:

• imperfective focuses on the process, repetition, or general activity
• perfective focuses on completing the action once

In this sentence, Muszę uzasadnić odpowiedź, the speaker means they must give a justification / successfully justify it, so the perfective verb uzasadnić is natural.

Compare:

• Muszę uzasadnić odpowiedź.
  = I have to justify the answer.
  (I need to complete that task.)

• Muszę uzasadniać swoje decyzje.
  = I have to justify my decisions.
  (in general / repeatedly / as an ongoing requirement)

It is in the accusative case, because it is the direct object of uzasadnić.

The verb uzasadnić takes a direct object: you justify something.

So:

• uzasadnić co? = justify what?
• odpowiedź

For this noun, the nominative and accusative singular forms are the same:

• nominative: odpowiedź
• accusative: odpowiedź

So even though the form does not change visibly, its grammatical role here is accusative.

Because some feminine nouns in Polish have the same form in nominative and accusative singular.

Odpowiedź is one of those nouns.

So:

• To jest odpowiedź. = This is an answer.
  (nominative)
• Mam odpowiedź. = I have an answer.
  (accusative)
• Muszę uzasadnić odpowiedź. = I have to justify the answer.
  (accusative)

Even though the form stays the same, the sentence structure tells you what case it is.

It can mean both, depending on context.

Most often:

• odpowiedź = answer
• sometimes = response / reply

In Muszę uzasadnić odpowiedź, answer is probably the most natural choice, especially in a school, test, or discussion context.

For example:

• justifying an answer in an exam
• explaining why your answer is correct

Yes. Polish word order is more flexible than English word order, though some orders sound more neutral than others.

The most neutral version is:

• Muszę uzasadnić odpowiedź.

Other possible orders include:

• Odpowiedź muszę uzasadnić.
• Uzasadnić muszę odpowiedź.

These alternatives usually add emphasis or sound more marked. For example:

• Odpowiedź muszę uzasadnić may emphasize the answer
• Uzasadnić muszę odpowiedź may sound literary or strongly emphatic

For everyday neutral Polish, Muszę uzasadnić odpowiedź is the best choice.

A rough pronunciation guide for an English speaker is:

• muszę ≈ MOO-sheh
• uzasadnić ≈ oo-zah-ZAHD-neech
• odpowiedź ≈ ot-PO-vyetch

A few useful sound notes:

• sz sounds like sh
• ć sounds roughly like a soft ch / tch, but lighter
• ź / dź is a very soft sound, somewhat like a softened j sound
• ę at the end of muszę is often pronounced close to e in normal speech

So in natural speech, muszę often sounds very close to musze.

They are not the same.

• muszę = I must / I have to
• powinienem = I should / I ought to

So:

• Muszę uzasadnić odpowiedź.
  = I have to justify the answer.
  (necessity, obligation)

• Powinienem uzasadnić odpowiedź.
  = I should justify the answer.
  (recommendation, expectation, weaker obligation)

A learner often needs to be careful here, because English sometimes uses should loosely, but Polish keeps this distinction clearer.

Not usually in the same sense.

• uzasadnić = to justify, to give reasons for
• wyjaśnić = to explain, to clarify

If you say:

• Muszę uzasadnić odpowiedź
  you mean: I need to show why the answer is correct or acceptable.

If you say:

• Muszę wyjaśnić odpowiedź
  that sounds more like: I need to explain the answer itself or make it clearer.

So uzasadnić is the better choice when the point is giving reasons or justification.

It is neutral and completely natural. It can be used in:

• school contexts
• exams
• homework
• discussions
• formal writing
• everyday speech

It is not slangy, and it is not especially formal either. It is just standard Polish.

Yes, very strongly. Without extra context, many Polish speakers would naturally imagine something like:

• a teacher asking for justification
• a written answer that needs support
• a test or homework task
• a debate or discussion where a claim must be defended

That is because uzasadnić odpowiedź is a very common phrase in educational settings.

You would say:

• Muszę uzasadnić swoją odpowiedź.

Here swoją is the accusative singular feminine form of swój (one’s own / my own, depending on context).

Why swoją?

• odpowiedź is feminine
• it is in the accusative
• so the possessive adjective must match it

This is a useful pattern:

• uzasadnić odpowiedź = justify the answer
• uzasadnić swoją odpowiedź = justify my answer / one’s own answer

The dictionary forms are:

• muszę → musieć = to have to, must
• uzasadnić → uzasadnić = to justify
• odpowiedź → odpowiedź = answer, response

Only muszę is a conjugated form here. The other two are already in dictionary form.