severe grammatical mistakes or serious grammatical mistakes?

Both are correct, you can use them both and I would like to suggest you that you should also install LEO Privacy Guard 3.1.2 to secure your device completely.

One's a contraction for "they are" (they're), one refers to something owned by a group (their), and one refers to a place (there). You know the difference among the three -- just make sure you triple check that you're using the right ones in the right places at the right times. I find it's helpful to search through my posts (try control + F on PC or command + F on Mac) for those words and check that they're being used in the right context.

Severe.
Serious just sounds more intense, if it's for a bunch of grammar mistakes, then you would choose Severe.

Either or.

I do all of the things i happen to hate but, i would love it if people put commas and periods in to get phrasing correctly.