Kad osjećam stres, mogu računati na svoje prijatelje.

Yes. Kad means when, and kada is the fuller form of the same word.

• Kad osjećam stres... = When I feel stress...
• Kada osjećam stres... means the same thing.

In everyday speech, kad is very common and natural. Kada can sound a bit more neutral or slightly more formal.

Osjećam is the 1st person singular present tense of osjećati (to feel).

So:

• ja osjećam = I feel
• ti osjećaš = you feel
• on/ona/ono osjeća = he/she/it feels

In the sentence, osjećam matches the understood subject I.

Croatian often drops subject pronouns when they are already clear from the verb ending.

• Osjećam already tells you the subject is I
• Mogu also already tells you the subject is I

So Ja kad osjećam stres, mogu računati na svoje prijatelje is possible, but ja is usually unnecessary unless you want emphasis or contrast.

This is a very common Croatian structure:

• mogu = I can
• računati = to count / to rely

After a modal verb like moći (can, to be able to), Croatian uses the infinitive.

So:

• mogu računati = I can count / rely
• mogu doći = I can come
• mogu pomoći = I can help

This works much like English can + verb.

Yes, and in this sentence it works like the English expression count on in the sense of rely on.

So računati na prijatelje does not mean mathematical counting here. It means:

• to rely on friends
• to depend on friends for support
• to know they will be there for you

This is a fixed and very common expression:

• računati na nekoga = to count on someone
• računati na nešto = to count on something

Because the expression računati na takes the accusative case, not the instrumental.

So:

• prijatelji = nominative plural
• prijatelje = accusative plural

That is why the sentence has:

• na svoje prijatelje

not

• na svojim prijateljima

The whole pattern is:

• računati na + accusative

Examples:

• Računam na tebe. = I’m counting on you.
• Računamo na pomoć. = We’re counting on help.

Because Croatian often uses the reflexive possessive svoj when the possessor is the same as the subject of the sentence.

Here, the subject is I, and the friends belong to that same subject:

• mogu računati na svoje prijatelje = I can count on my own friends

Using svoj is the normal Croatian choice here.

You may sometimes hear moje prijatelje, but svoje prijatelje is usually more natural and more standard in this kind of sentence.

A useful rule:

• If the owner is the same as the subject, svoj is often preferred.

Svoje is the accusative plural form of svoj, because it agrees with prijatelje.

So the phrase is:

• svoje prijatelje
  both words are plural, masculine animate, accusative

This agreement is very important in Croatian: adjectives and possessive words change to match the noun they describe.

Prijatelj means friend and is a masculine animate noun.

In Croatian, masculine animate nouns usually have:

• nominative plural: prijatelji
• accusative plural: prijatelje

Because računati na requires the accusative, you get:

• na prijatelje
• na svoje prijatelje

This masculine animate pattern is something learners see a lot:

• vidim prijatelje = I see friends
• znam ljude = I know people
• čekam goste = I’m waiting for guests

Yes, osjećam stres is understandable and natural enough. But Croatian also often expresses this idea in other ways, for example:

• Kad sam pod stresom... = When I’m under stress...
• Kad osjećam stres... = When I feel stress...
• Kad sam pod pritiskom... = When I’m under pressure... depending on context

So your sentence is correct, but in everyday language many speakers might especially like pod stresom sam.

Croatian word order is fairly flexible, though some versions sound more natural than others.

The original:

• Kad osjećam stres, mogu računati na svoje prijatelje.

Possible variations:

• Mogu računati na svoje prijatelje kad osjećam stres.
• Na svoje prijatelje mogu računati kad osjećam stres.

These all mean basically the same thing, but the focus changes slightly.

The original version is very natural because it starts with the time clause Kad osjećam stres and then gives the main statement.

The tricky part is ć.

• ć is a soft sound, somewhat like a very soft ch/t sound
• It is different from č, which is usually stronger/harder

In osjećam, try to hear it as roughly:

• o-sye-cham, but with a softer ch than English usually has

Also:

• je in osjećam is pronounced as part of the word, not like a separate syllable in English spelling

A rough learner-friendly pronunciation is:

• kahd o-SYE-cham stres, MOH-goo ra-CHOO-nah-tee nah SVO-ye pri-YA-te-lye

That is only approximate, but it can help at first.

Usually, kad is better here.

• kad = when
• ako = if

Your sentence means that in the situation when you feel stress, you can rely on your friends. That is why kad fits naturally.

If you said:

• Ako osjećam stres, mogu računati na svoje prijatelje

it would sound more like If I feel stress, I can count on my friends. That is not impossible, but it changes the meaning slightly from a general when this happens to a more conditional if this happens.