Learn Hardcore Czech: Když máš otázku, můžeš mi zavolat na telefon a já ti odpovím. - When you have a question, you can call me on the phone and I will answer you.

So Když máš otázku… literally looks like “When you have a question…”, but in context it means “When(ever)/If you have a question (in the future)…”.

Using budeš mít would sound unusual here. The natural pattern is:

Když máš čas, zavolej mi. – When/If you have time, call me.
Když prší, zůstanu doma. – When it rains, I stay at home.

So: present tense in the když-clause, even though it refers to the future.

Why is it otázku and not otázka?

Otázka is a feminine noun.

otázka – nominative singular (subject form, “a question”)
otázku – accusative singular (direct object form, “a question” as object)

In the sentence, máš otázku = “you have a question”, and otázku is the object of the verb máš (have), so it must be in the accusative:

Otázka je těžká. – The question is hard. (subject → nominative)
Mám otázku. – I have a question. (object → accusative)

What is the difference between když and jestli here? Could I say Jestli máš otázku…?

Both can appear in similar contexts, but there’s a nuance:

když = when(ever) – more neutral, can sound like a general rule or a repeated situation.
jestli = if / whether – emphasizes uncertainty (maybe you will, maybe you won’t).

In this sentence:

Když máš otázku, můžeš mi zavolat…
= Whenever / When you have a question, you can call me.
Jestli máš otázku, můžeš mi zavolat…
= If you have a question, you can call me (if not, then never mind).

Both are grammatically correct here. Když sounds a bit more like a standing offer, a general instruction.

Why is it můžeš mi zavolat and not můžeš zavolat mi?

Czech has a fairly fixed position for short pronouns like mi, ti, mu, se. They are clitics and usually occupy the second position in the clause.

So we normally say:

Můžeš mi zavolat. ✔
Můžeš zavolat mi. ✘ (sounds wrong / very unnatural)

More examples:

Zítra ti napíšu. – I’ll write to you tomorrow.
Potom mu to vysvětlím. – Then I’ll explain it to him.

In each sentence, the short pronoun (ti, mu, mi) comes right after the first stressed element in the clause.

Is na telefon necessary? What is the difference between můžeš mi zavolat and můžeš mi zavolat na telefon?

Na telefon literally means “on the phone”, but in everyday Czech it’s often redundant because zavolat (někomu) already implies calling by phone.

Můžeš mi zavolat. – You can call me. (normally understood as “call me on the phone”)
Můžeš mi zavolat na telefon. – You can call me on the phone. (slightly more explicit, but not really needed)

Czech speakers often just say:

Když máš otázku, zavolej mi.
Když budeš něco potřebovat, můžeš mi zavolat.

So yes, můžeš mi zavolat alone is completely natural and often preferred.

Why is it můžeš mi zavolat na telefon and not můžeš mi zavolat po telefonu?

Both exist, but they’re used differently:

na telefon – focuses more on the device (“on my phone”); in common speech it mostly just sounds like “by phone” but is a bit redundant.
po telefonu – more literally “by phone / over the phone”, emphasizing the medium rather than the device.

Typical uses:

Můžeš mi zavolat. – You can call me. (most natural)
Můžeme to vyřešit po telefonu. – We can solve it by phone.
Napiš mi e‑mail nebo zavolej na telefon. – Write me an email or call this phone number. (here na telefon can refer to a specific phone number)

In your sentence, po telefonu would sound odd. The simplest and most natural is just můžeš mi zavolat.

What is the difference between mi and ti in this sentence?

Both mi and ti are dative forms of pronouns, used for indirect objects (“to me”, “to you”).

mi = to me (1st person singular)
ti = to you (2nd person singular, informal)

In the sentence:

můžeš mi zavolat = you can call me (literally “you can call to-me”)
já ti odpovím = I will answer you (literally “I will answer to-you”)

So the pattern is:

Zavoláš mi a já ti odpovím. – You’ll call me and I’ll answer you.

Why do we use ti (informal) and not a formal form like vám?

Czech distinguishes informal (T) and formal (V) address forms.

ty / ti / tě = informal singular “you”
vy / vám / vás = formal singular “you” or plural “you all”

Your sentence uses informal singular:

máš (you have, informal)
můžeš (you can, informal)
ti (to you, informal)

A formal version would be:

Když máte otázku, můžete mi zavolat na telefon a já vám odpovím.

So the original sentence is clearly speaking to one person you are on “ty” terms with (friend, family member, colleague you know well, etc.).

Why is odpovím used (future perfective) instead of odpovídám or budu odpovídat?

Czech uses verb aspect (perfective vs imperfective) to express different kinds of actions, especially in the future:

odpovídat – imperfective: “to be answering, to answer repeatedly/regularly”
odpovědět / odpovím – perfective: “to answer (once, to give a response)”

Here, odpovím is perfective future and means “I’ll give you an answer (I’ll respond)” – a single, complete act.

If you said:

a já ti odpovídám – “and I am answering you” (ongoing present, not about the future)
a já ti budu odpovídat – “and I will (be) answering you / I will keep answering you” (more about a repeated or ongoing activity)

For the idea “I will answer you (once)”, odpovím is the natural choice.

Is the pronoun já in a já ti odpovím necessary? Could we just say a ti odpovím?

The subject pronoun já is not strictly necessary in Czech; the verb ending ‑ím already shows it’s “I”. However:

A odpovím ti. – fully correct and natural.
A já ti odpovím. – also correct, with a bit more emphasis on “I”.

Adding já can:

make the structure parallel to ty (you): Ty mi zavoláš a já ti odpovím.
emphasize the contrast: You call me, and I (in return) will answer you.

But “a ti odpovím” by itself is wrong, because ti is dative (“to you”), not nominative (“you”). The subject would be missing if you removed já.

Why is there a comma after Když máš otázku?

In Czech, a subordinate clause (introduced by když, protože, že, etc.) is usually separated from the main clause by a comma.

Here:

Když máš otázku – subordinate clause (“when/if you have a question”)
můžeš mi zavolat na telefon a já ti odpovím – main clause (“you can call me on the phone and I will answer you”)

So the rule is: clause (když…) , main clause:

Když mám čas, přijdu.
Když prší, zůstanu doma.

Could we change the word order in the second part? For example, a odpovím ti instead of a já ti odpovím?

Yes, both are correct, with slightly different emphasis:

a já ti odpovím – more neutral; mild emphasis on já (= I).
a odpovím ti – slightly more neutral/prosodic; the focus can shift more to the verb/action (odpovím).

What you generally cannot do is move ti freely:

a odpovím ti ✔ (clitic in second position after odpovím within that mini‑clause)
a odpovím na otázku ti ✘ (wrong position of ti)
a ti odpovím ✔ but here ti is again in second position of the clause: a | ti | odpovím. This is also grammatical and can sound a bit more emphatic on “to you”.

Czech allows relatively flexible word order, but clitics (mi, ti, se, si, ho, jí, etc.) tend to stay in or near the second position in their clause.