"Kommer" is indeed the present tense, and "kom" is the preterite tense.
Å komme - kommer - kom - har kommet
Hvem kom? => Who came?
Kom de med fly? => Did they travel by plane?
The uses of "kom" in these sentences can be changed to "kommer", which also would change their temporal aspect.
Hvem kommer? => Who's coming?
Kommer de med fly? => Are they travelling by plane?
God jul!