Muszę zraszać roślinę doniczkową.

Because Polish usually drops subject pronouns when they are not needed. The verb form muszę already tells you the subject is I.

• muszę = I must / I need to
• ja muszę is possible, but it adds emphasis, like I need to, not someone else.

So Muszę zraszać roślinę doniczkową is the normal, natural version.

Muszę is the 1st person singular form of the verb musieć.

• musieć = to have to / must / need to
• muszę = I have to / I must

A few useful forms:

• muszę = I must
• musisz = you must
• musi = he/she/it must
• musimy = we must
• musicie = you all must
• muszą = they must

After musieć, Polish uses the infinitive of the main verb.

So the pattern is:

• muszę czytać = I have to read
• muszę pracować = I have to work
• muszę zraszać = I have to mist/spray

You do not use a second conjugated verb here. So muszę zraszam would be wrong.

Yes. Zraszać is imperfective.

In this sentence, the imperfective suggests the action as:

• an activity in general,
• something repeated,
• or something you need to do as a task, without focusing on its completion.

So Muszę zraszać roślinę doniczkową sounds like:

• I need to mist the plant,
• I’m supposed to be doing that,
• or it’s something that needs doing, perhaps regularly.

A perfective counterpart is zrosić:

• Muszę zrosić roślinę doniczkową = I need to mist the plant once, as a completed action.

They are related, but not the same.

• zraszać = to mist, spray lightly with water, usually on the leaves or surface
• podlewać = to water, usually by pouring water into the soil

So for a houseplant:

• podlewać roślinę = water the plant
• zraszać roślinę = mist the plant

English sometimes just says water the plant, but Polish distinguishes these actions more clearly.

Because the plant is the direct object of the verb zraszać, so it has to be in the accusative case.

The base form is:

• roślina doniczkowa = a potted plant / houseplant

In this sentence it changes to:

• roślinę doniczkową

Both the noun and the adjective change because they must agree with each other.

So:

• roślina → roślinę
• doniczkowa → doniczkową

It is accusative singular feminine.

Why?

• roślina is a feminine noun
• it is singular
• it is the direct object of zraszać

That is why the endings are:

• noun: -ę
• adjective: -ą

This is a very common pattern with feminine singular nouns:

• widzę kobietę = I see a woman
• mam dobrą książkę = I have a good book
• zraszam roślinę doniczkową = I mist a potted plant

That is a great question, because many Polish adjectives do often come before the noun. But some combinations are commonly used with the adjective after the noun, especially when the adjective is classifying the noun rather than simply describing it.

Roślina doniczkowa is a very standard expression meaning a houseplant / potted plant.

So here the phrase works almost like a fixed category name:

• roślina doniczkowa = a plant of the “potted” type

If you said doniczkowa roślina, people would understand it, but it can sound less neutral or more stylistically marked than the usual roślina doniczkowa.

Yes. Polish word order is flexible, because endings show grammatical relationships.

The neutral order here is:

• Muszę zraszać roślinę doniczkową.

But other orders are possible for emphasis:

• Roślinę doniczkową muszę zraszać.
  This emphasizes the potted plant.

• Muszę roślinę doniczkową zraszać.
  Also possible, especially in speech, though less neutral.

For a learner, the original version is the safest and most natural default.

A rough English-friendly approximation is:

MOO-sheh ZRA-shach rosh-LEE-neh doh-NEECH-KOH-voh

A few pronunciation notes:

• muszę: sz sounds like English sh
• zraszać begins with zr-, a real z sound followed by r
• ś in roślinę is a softer sound than sz
• cz in doniczkową sounds like ch in church
• final ę is often pronounced less strongly nasal than learners expect

If you like IPA, a careful pronunciation is approximately:

/ˈmu.ʂɛ ˈzra.ʂat͡ɕ rɔɕˈli.nɛ dɔˈɲit͡ʂ.kɔ.vɔ̃/

Yes. If you want to say this potted plant, you can say:

• Muszę zraszać tę roślinę doniczkową.

Here tę is the accusative singular feminine form of ta (this), and it agrees with roślinę.

So:

• ta roślina doniczkowa = this potted plant
• tę roślinę doniczkową = this potted plant (as the object)

This is useful when you mean one specific plant, not just a plant in general.