Learn Icelandic: Ég kem með þér.

The preposition með means "with". When it is used to talk about accompanying someone, it requires the following word to be in the dative case. Therefore, the basic pronoun þú (you) changes to its dative form, þér.

How do I know if ég kem means "I come" or "I am coming"?

Icelandic does not have a separate "am doing" tense for action verbs like coming and going. Ég kem covers both "I come" and "I am coming". You just rely on the context of the conversation to know which one makes sense in English.

Will the preposition með always make the next word dative?

Not always! When með means accompanying someone (going together), it takes the dative case, as it does in this sentence. However, if it means you are physically carrying or bringing an object, it takes the accusative case instead.

Why does the verb kem look different from its dictionary form?

It is an irregular verb. It comes from the basic verb koma (to come). The vowel changes from an "o" to an "e" for the "I" form, which is why it becomes ég kem.