Musíme zalévat sazenice.

Because Czech usually leaves out subject pronouns when the verb form already shows who the subject is.

Musíme is the 1st person plural form of muset, so it already means we must / we have to.
If you want extra emphasis, you can add my:

My musíme zalévat sazenice. = We have to water the seedlings.

That sounds more contrastive or emphatic, not neutral.

The sentence breaks down like this:

• Musíme = we must / we have to
• zalévat = to water
• sazenice = seedlings

So the structure is:

muset + infinitive + object

That is a very common Czech pattern, just like English must water something.

Because after a modal verb like muset, Czech normally uses the infinitive.

So:

• Musíme pracovat. = We have to work.
• Musím jít. = I have to go.
• Musíme zalévat sazenice. = We have to water the seedlings.

This works very much like English must + verb.

Musíme is the present-tense we form of muset.

A few forms of muset are:

• musím = I must
• musíš = you must
• musí = he/she/it must
• musíme = we must
• musíte = you must
• musí = they must

So the ending -íme tells you the subject is we.

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

• zalévat is imperfective
• zalít is perfective

In simple terms:

• zalévat focuses on the action as ongoing, repeated, or habitual
• zalít focuses on one completed act

So:

• Musíme zalévat sazenice. suggests a general duty or repeated action, such as regularly watering seedlings.
• Musíme zalít sazenice. would suggest that the seedlings need one complete watering now.

English does not mark this contrast as clearly, so this is a very common question for learners.

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

We are watering what?
Answer: sazenice

So the noun must be in the object case.

Here it is accusative plural.

Mostly from meaning and context.

The form sazenice can be tricky because it can look the same in more than one grammatical role. In this sentence, it means seedlings, so it is understood as accusative plural.

A useful comparison:

• sazenice = nominative singular or nominative plural or accusative plural, depending on context
• sazenici = accusative singular

So:

• Musíme zalévat sazenici. = We have to water one seedling.
• Musíme zalévat sazenice. = We have to water seedlings.

Yes. Czech word order is flexible, and changing it usually changes emphasis, not the basic meaning.

Neutral word order:

Musíme zalévat sazenice.

Other possible orders:

• Musíme sazenice zalévat.
• Sazenice musíme zalévat.

These all mean roughly the same thing, but the focus shifts:

• starting with sazenice puts more emphasis on the seedlings
• keeping zalévat sazenice together feels more neutral

For a learner, Musíme zalévat sazenice. is a perfectly natural basic version.

It can match both must and have to, depending on context.

Musíme expresses necessity or obligation, but Czech does not always separate strong inner obligation and outside necessity the same way English sometimes does.

So it could mean:

• we are required to do it
• we need to do it
• circumstances make it necessary

In many everyday situations, English have to is often the most natural translation, even though must is also correct.

There are two very important possibilities:

• Nemusíme zalévat sazenice. = We don’t have to water the seedlings.
• Nesmíme zalévat sazenice. = We mustn’t / are not allowed to water the seedlings.

This is a key difference for English speakers:

• nemusíme = no necessity
• nesmíme = prohibition

They are not interchangeable.

A few useful pronunciation points:

• Czech stress is normally on the first syllable of each word.
• í and é are long vowels.
• c is pronounced like ts.

So roughly:

• MU-si-me
• ZA-le-vat
• SA-ze-ni-tse

More naturally, you would say the long vowels clearly in musíme and zalévat.

You do not need to reduce unstressed vowels the way you often do in English. Czech vowels stay fairly clear.