Počet ćemo raditi kad dođeš.

It’s the simple future (Futur I). Croatian forms Futur I with:

• the infinitive of the main verb, plus
• the clitic forms of htjeti: ću, ćeš, će, ćemo, ćete, će.

So: početi + ćemo → počet ćemo = we will start.

When the future clitic (ću/ćeš/će/ćemo/ćete/će) immediately follows the infinitive, the final -i of the infinitive drops:

• Correct: Počet ćemo (not Početi ćemo)
• Correct: Radit ću (not Raditi ću)
• Correct: Vidjet ćeš (not Vidjeti ćeš)

If the clitic does not immediately follow, keep the full infinitive:

• Mi ćemo početi raditi.
• Večeras ćemo početi.

Clitics like ću/ćeš/će/ćemo/ćete/će sit in the “second position” of the clause (after the first stressed word/phrase):

• Počet ćemo raditi...
• Mi ćemo početi raditi... (here the clitic follows Mi)
• Kad dođeš, počet ćemo raditi. They cannot start a sentence by themselves.

In future time clauses with kad/kada, Croatian uses the present tense (often of a perfective verb):

• Počet ćemo raditi kad dođeš.

Using the future there (kad ćeš doći) is nonstandard in Croatian (though heard regionally). Note: Kad ćeš doći? is correct as a standalone question (“When will you come?”).

Yes. Kad is shorter and very common; kada is a bit more formal/careful. Meaning and grammar are identical:

• Počet ćemo raditi kad/kada dođeš.

Yes. That’s Futur II (future perfect) in the time clause: budeš (future of biti) + past participle došao. It’s more formal and emphasizes completion of your arrival before the starting begins. Everyday speech often uses the simpler kad dođeš.

• Masculine: Kad budeš došao, ...
• Feminine: Kad budeš došla, ...
• Formal/plural: Kad budete došli, ...

Yes. If the kad/kada clause comes first, use a comma:

• Kad dođeš, počet ćemo raditi.

If it comes last, no comma:

• Počet ćemo raditi kad dođeš.

Use vi-forms:

• Počet ćemo raditi kad dođete. In very formal writing you can capitalize: kad Vi dođete.

Negate the future auxiliary, and for “until,” use dok ne:

• Nećemo početi raditi dok ne dođeš. = We won’t start working until you come.

A plain negation with kad is possible but less natural for this meaning:

• Nećemo početi raditi kad dođeš.

Yes. Adding mi emphasizes “we.” Because ćemo now attaches to mi, the main verb appears as the full infinitive:

• Mi ćemo početi raditi kad dođeš. (emphasis on “we”)
• Počet ćemo raditi kad dođeš. (neutral)

Raditi = “to work” (imperfective, ongoing activity). Uraditi/napraviti = “to do/complete (a task)” (perfective). After početi, you normally use an activity verb:

• početi raditi (start working)
• početi pisati (start writing) Početi uraditi isn’t idiomatic.

Yes. Počinjemo (imperfective present) is used for scheduled/near-future plans or habitual statements:

• Scheduled: Počinjemo raditi kad dođeš (sutra u 9).
• Habitual: Počinjemo raditi kad dođeš (every time you arrive, we start). For a one-off future, Počet ćemo is the straightforward choice.

Often, yes. Two natural options:

• Započet ćemo raditi kad dođeš.
• More idiomatic with a noun: Započet ćemo s radom kad dođeš. Both mean “we will begin/start,” with početi being the default, all-purpose verb.