Ako otkažeš svoj plan, javi se i pridruži nam se.

Because after ako meaning if, Serbian normally does not use the imperative.

Here, otkažeš is the 2nd person singular present tense of the perfective verb otkazati. After ako, this present form is often used for a future condition, so it means something like if you cancel.

So:

• ako otkažeš = if you cancel
• otkaži! = cancel!

Those are different functions.

Svoj is the reflexive possessive pronoun, and Serbian prefers it when the thing belongs to the subject of the sentence.

Here, the subject is an understood you, so:

• svoj plan = your own plan

Using tvoj plan is not impossible in everyday speech, but svoj is the standard and more natural choice when the possessor is the same person as the subject.

A useful rule:

• use svoj when the owner = the subject
• use moj / tvoj / njegov / njen... when you want to name some other owner, or for emphasis

Javiti se is a very common Serbian verb that literally has a reflexive form, but in real usage it often means:

• get in touch
• let someone know
• call/message
• check in

So in this sentence, javi se is best understood as something like let us know or contact us.

It is one of those verbs you should learn as a whole expression:

• javiti se = to get in touch / to make contact

Grammatically, se is the reflexive particle, but in many Serbian verbs it is just part of the normal dictionary form and does not feel strongly reflexive in English.

So although javiti se may historically look like announce oneself, in modern usage you should mostly learn it as a fixed verb:

• javiti = announce / report / let know something
• javiti se = get in touch / make contact / respond

In other words, you usually should not drop se here.

Because pridružiti se is also a verb that normally comes with se.

It means to join in the sense of join someone / become part of their group or activity.

So:

• pridružiti se nekome = to join someone

Again, this is best learned as a complete verb:

• pridružiti se = to join

Without se, you would not have the same normal meaning.

Nam means to us and is the dative form of mi.

The full stressed form is nama, while nam is the unstressed clitic form.

With pridružiti se, Serbian uses the dative:

• pridružiti se kome? = join whom?
• pridružiti se nama / nam = join us

So pridruži nam se literally works like join yourself to us, though the natural English meaning is simply join us.

Because nam and se are both clitics: short unstressed words that have special placement rules in Serbian.

In imperative sentences, clitics usually come right after the imperative verb, and when several clitics appear together, they follow a usual order. Here the natural order is:

• pridruži nam se

not:

• pridruži se nam

You will see the same pattern in other common phrases:

• javi mi se = contact me
• pridruži nam se = join us
• predstavi mi se = introduce yourself to me

So this is not random word order; it follows standard clitic placement.

Yes. They are both 2nd person singular imperatives, used to speak directly to one person.

• javi se = get in touch
• pridruži nam se = join us

So the structure of the whole sentence is:

• Ako otkažeš svoj plan = conditional clause
• javi se i pridruži nam se = two commands/invitations

Because Serbian often omits subject pronouns when the verb ending already shows the person clearly.

Here:

• otkažeš = you cancel
• javi = command to you
• pridruži = command to you

So ti is unnecessary unless you want emphasis or contrast.

For example:

• Ako ti otkažeš svoj plan... could sound contrastive, like if you are the one who cancels your plan...

But in the normal sentence, leaving out ti is natural.

Yes, it could be plural, but the singular is natural if the speaker means the plan/arrangement you currently have.

So:

• svoj plan = your plan / your arrangement
• svoje planove = your plans

The singular often sounds like one specific prior commitment. The plural would suggest several plans or a more general schedule.

Yes, you could, especially if the context already makes it obvious whose plan is being discussed.

But svoj makes the ownership explicit and sounds very natural because the plan belongs to the subject.

So:

• otkažeš plan = possible, a bit less explicit
• otkažeš svoj plan = clearer, standard, and very natural

It is from the perfective verb otkazati.

That matters because Serbian often uses a perfective present form after ako when talking about a future completed action:

• Ako otkažeš... = If you cancel... / If you end up canceling...

The action is seen as a single completed event.

If you used an imperfective verb instead, the meaning would usually shift toward repeated, ongoing, or habitual action, which is not what this sentence wants.

So otkažeš fits the idea of one specific cancellation very well.