Learn Hardcore Croatian: Tvoja kolegica i moj kolega su dobar tim. - Your female colleague and my male colleague are a good team.

kolegica = female colleague → feminine, singular, nominative
The possessive for ti (you, informal) is:
- tvoj for masculine singular nouns
- tvoja for feminine singular nouns
- tvoje for neuter singular nouns

Since kolegica is feminine singular, you must say tvoja kolegica.

If the noun were masculine, for example kolega (male colleague), you would say tvoj kolega.

Why is it moj kolega and not moja kolega?

Even though kolega ends in -a, it is grammatically masculine (like muškarac = man; sudija in some dialects). So the possessive must also be masculine.

kolega = male colleague → masculine, singular, nominative
The possessive for ja (I) is:
- moj for masculine singular nouns
- moja for feminine singular nouns
- moje for neuter singular nouns

Therefore:

moj kolega = my (male) colleague
moja kolegica = my (female) colleague

Why is the verb su and not je?

Su is the 3rd person plural form of the verb biti (to be), present tense.

je = he/she/it is (3rd person singular)
su = they are (3rd person plural)

The subject here is Tvoja kolegica i moj kolega – that is two people, so the subject is plural. Croatian verbs must agree with the subject in number, so you use su, not je:

Tvoja kolegica je… = Your (female) colleague is…
Moj kolega je… = My (male) colleague is…
Tvoja kolegica i moj kolega su… = Your colleague and my colleague are…

Why is it dobar tim and not dobri tim or dobar timovi?

Tim (team) is a singular masculine noun here, so the adjective must also be masculine singular:

dobar (good) – masculine singular
dobra – feminine singular
dobro – neuter singular
dobri – masculine plural
dobre – feminine plural
dobra – neuter plural

You are saying that together they form one team, so Croatian uses singular:

su dobar tim = (they) are a good team

If you were talking about several teams, you would pluralize the noun and the adjective:

(Oni) su dobri timovi. = They are good teams.

Why is the verb plural (su) but tim is singular?

There are two different things here:

The subject: Tvoja kolegica i moj kolega → two people → plural
→ the verb must be plural: su
The complement (predicative noun): dobar tim
This describes what they are together – they form one team → singular.

This kind of mismatch between plural subject and singular complement is completely normal in Croatian (and in English too: They are a good team).

You can think of an implicit phrase:

Tvoja kolegica i moj kolega zajedno su dobar tim.
Your colleague and my colleague together are a good team.

Could I say Tvoja kolegica i moj kolega su dobri tim?

No, that would be ungrammatical.

tim is singular, so it cannot take dobri, which is plural.
The adjective dobar must agree with tim, not with the two people individually.

So you say:

Oni su dobar tim. = They are a good team.
Not Oni su dobri tim.

Why is there no word for “a” in dobar tim (a good team)?

Croatian has no articles (no a/an and no the).
The noun phrase dobar tim can mean:

a good team
the good team
sometimes just good team (in a general sense)

The exact meaning (a/the) is usually clear from the context, not from any word in the sentence. Here the most natural English translation is a good team.

What is the difference between kolegica / kolega and prijateljica / prijatelj?

kolegica / kolega = colleague (someone you work or study with; a professional or school relationship)
prijateljica (f) / prijatelj (m) = friend (personal relationship)

So:

Tvoja kolegica i moj kolega su dobar tim.
→ Your (female) colleague and my (male) colleague are a good team.

If you said:

Tvoja prijateljica i moj prijatelj su dobar tim.
→ Your (female) friend and my (male) friend are a good team.

…it would emphasize friendship, not the workplace/school context.

Can I change the word order? For example: Moj kolega i tvoja kolegica su dobar tim?

Yes, word order in Croatian is relatively flexible, especially for subjects like this. Both are correct:

Tvoja kolegica i moj kolega su dobar tim.
Moj kolega i tvoja kolegica su dobar tim.

The difference is mainly in emphasis or in what you mention first. Grammatically, both are fine.

You could also move the predicate:

Tvoja kolegica i moj kolega su tim, i to dobar.
(Your colleague and my colleague are a team, and a good one at that.)

…but Tvoja kolegica i moj kolega su dobar tim is the most neutral version.

How would I say this politely (formal your) instead of informal tvoja?

For formal you (addressing one person politely, or a group), you use vaš/vaša/vaše instead of tvoj/tvoja/tvoje.

Since kolegica is feminine singular, you choose vaša:

Vaša kolegica i moj kolega su dobar tim.
Your (female) colleague and my (male) colleague are a good team.
(formal/polite your)

How would I make this sentence negative (say they are not a good team)?

To negate the verb biti (to be), you add ne in front of the verb form, and it usually merges into one word:

nisam, nisi, nije, nismo, niste, nisu

So su → nisu:

Tvoja kolegica i moj kolega nisu dobar tim.
Your colleague and my colleague are not a good team.

How do you pronounce the sentence Tvoja kolegica i moj kolega su dobar tim?

Approximate pronunciation with English-like hints (stressed syllables in CAPS):

TVO-ja – TVO like tvo in tvoy, j is like English y
ko-LE-gi-ca – ko like co in cold, LE stressed, gi like ghee
i – like ee in see
moj – like moy (rhymes with boy)
ko-LE-ga – same as kolegica, but ending ga
su – like soo
DO-bar – DO stressed, as in door without the r
tim – like English team, but shorter i

So:
TVO-ja ko-LE-gi-ca i moj ko-LE-ga su DO-bar tim.

What are the dictionary/base forms of the words in this sentence?

All the content words here are already in their nominative singular (base) forms, except the possessives, which are inflected:

tvoja → base possessive: tvoj (your, informal)
kolegica → kolegica (female colleague)
i → i (and)
moj → moj (my)
kolega → kolega (male colleague)
su → infinitive: biti (to be); su is 3rd person plural, present
dobar → base adjective: dobar (good)
tim → tim (team)