Questions & Answers about Ist jemand in das Zimmer gekommen?
The subject here is jemand (someone). Does this word change its ending in other sentences?
It can, but it doesn't always! Here, jemand is in the nominative case because it is the subject performing the action. If it were the object of a sentence (like "I see someone"), you can add an ending: jemanden (accusative) or jemandem (dative). However, in everyday spoken German, people often just leave it as jemand in all cases.
Could I use irgendjemand instead of jemand in this sentence?
Yes, absolutely! Adding the prefix irgend- just makes the pronoun more vague or random. While jemand means "someone", irgendjemand emphasizes "anyone at all". Both work perfectly well here.
Why do we use ist instead of hat for the past tense here?
We use ist (is) because the main verb is kommen (to come). In the spoken past tense (Perfekt), verbs that describe a change of location or a change of state must use a form of sein (to be) as their helping verb instead of haben (to have).
Why is it in das Zimmer and not in dem Zimmer (or im Zimmer)?