Kad imam gripu, ostajem kod kuće i pijem puno vode.

Yes. Kad means when here.

In this sentence, Kad imam gripu means When I have the flu.

A learner should also know that Croatian often uses both kad and kada. They usually mean the same thing, but:

• kad is more common in everyday speech
• kada can sound a bit more formal or emphatic

So you could also say:

• Kada imam gripu, ostajem kod kuće i pijem puno vode.

Both are correct.

Because the verb imati (to have) takes a direct object, and the direct object is usually in the accusative case.

The noun is:

• gripa = the flu (dictionary form / nominative)

But after imam (I have), it changes to the accusative:

• gripu

So:

• imam gripu = I have the flu

This is very common in Croatian:

• Imam knjigu. = I have a book.
• Imam sestru. = I have a sister.
• Imam gripu. = I have the flu.

Imam is the 1st person singular present tense of imati (to have).

So:

• ja imam = I have

Croatian often drops subject pronouns when they are not needed, because the verb ending already shows the person. That is why the sentence does not need ja.

You could say:

• Kad ja imam gripu...

but normally Croatian prefers:

• Kad imam gripu...

because imam already clearly means I have.

Ostajem is the present tense of ostajati / ostati, and here it means I stay or I remain.

In this sentence:

• ostajem kod kuće = I stay at home

Croatian present tense can often translate into English in more than one way depending on context:

• I stay at home
• I am staying at home

Here, because the sentence describes what you generally do when you have the flu, the best English translation is usually:

• I stay at home

Kod kuće is a very common Croatian expression meaning at home.

So:

• ostajem kod kuće = I stay at home

This expression is idiomatic, and learners should memorize it as a set phrase.

Compare:

• kod kuće = at home
• u kući = in the house

These are not always the same. U kući focuses more on being physically inside a house/building, while kod kuće means at home in the usual everyday sense.

In the expression kod kuće, the word kuće is in the genitive case.

That is because the preposition kod normally takes the genitive.

So:

• kuća = house, home
• kod kuće = at home

This is worth learning as a fixed phrase, but it also shows an important grammar pattern:

• kod + genitive

For example:

• kod prijatelja = at a friend’s place
• kod liječnika = at the doctor’s / at the doctor

Pijem means I drink and is the 1st person singular present tense of piti (to drink).

So:

• pijem = I drink / I am drinking

In the sentence:

• pijem puno vode = I drink a lot of water

Again, Croatian does not need the pronoun ja, because pijem already tells you the subject is I.

Because puno (a lot of) is followed by the genitive case.

The noun is:

• voda = water

But after puno, it becomes:

• vode

So:

• puno vode = a lot of water

This is a very useful pattern:

• puno vremena = a lot of time
• puno ljudi = a lot of people
• puno novca = a lot of money
• puno vode = a lot of water

Because Croatian commonly uses the present tense to describe repeated, usual, or general actions.

So this sentence means something like:

• When I have the flu, I stay at home and drink a lot of water.

It describes a habit or usual response, not only something happening right now.

This is very natural in Croatian and works much like English present tense in general statements.

Croatian often does not need an extra word like then when the meaning is already clear.

The structure:

• Kad imam gripu, ostajem kod kuće i pijem puno vode.

already naturally means:

• When I have the flu, I stay at home and drink a lot of water.

If you translated very literally, you might expect something like then I stay..., but Croatian usually just continues with the main clause directly.

Yes, Croatian word order is more flexible than English, although some orders sound more natural than others.

The original sentence:

• Kad imam gripu, ostajem kod kuće i pijem puno vode.

is very natural.

You can also put the main clause first:

• Ostajem kod kuće i pijem puno vode kad imam gripu.

This still means the same thing: I stay at home and drink a lot of water when I have the flu.

The difference is mostly one of emphasis and flow, not basic meaning.

Because Kad imam gripu is a subordinate clause at the beginning of the sentence, and Croatian normally separates it from the main clause with a comma.

So:

• Kad imam gripu, ostajem kod kuće i pijem puno vode.

This is similar to English:

• When I have the flu, I stay at home and drink a lot of water.

The comma helps show where the introductory clause ends and the main clause begins.

Yes. Gripa is the standard everyday word for flu.

In this sentence it appears as gripu because of the accusative case after imam.

So remember:

• gripa = flu
• imam gripu = I have the flu

A learner may also hear other illness expressions with imati, for example:

• Imam temperaturu. = I have a fever.
• Imam prehladu. = I have a cold.

Yes, that would also be correct, but it means something slightly broader.

• Kad imam gripu = When I have the flu
• Kad sam bolestan = When I am sick

So the original sentence is specifically about the flu, while your version is about being ill in general.

Also note:

• bolestan is masculine singular
• a female speaker would say bolesna

So:

• Kad sam bolestan... = when I am sick (male speaker)
• Kad sam bolesna... = when I am sick (female speaker)