Learn Swedish: Hennes chef kommer imorgon. - Her boss is coming tomorrow.

It doesn't! Unlike words like min (my) which change to mitt or mina, the third-person possessive pronouns (hennes, hans, dess, and deras) never change their form. Hennes stays exactly the same no matter what type of noun comes after it.

Why is the noun chef in the indefinite form here instead of the definite form (chefen)?

Whenever you use a possessive pronoun like hennes (her) before a noun, the noun must be in the indefinite form. Just like in English where you wouldn't say "her the boss", in Swedish you use hennes chef and never hennes chefen.

The translation says "is coming", but the Swedish sentence just uses the present tense kommer. Why isn't there an "is" word?

Swedish doesn't have a continuous "-ing" tense like English does. The regular present tense verb kommer covers both "comes" and "is coming". Trying to translate "is coming" word-for-word (such as adding är before the verb) is a very common beginner mistake, so just remember that one Swedish verb does the job for both!

Is it correct to write imorgon as one word, or should it be two words (i morgon)?