Will a 30-day COVID extension affect future visa applications and how long can I stay outside Thailand on a student visa?

just ignore these answers.

1. Will getting a 30-day covid extension of visa on arrival making getting tourist and student visa later more difficult? - you need to talk that with the institution who would provide you the ED visa. it might or not, depending on the officer who approves the visa or your past in Thailand

2. When you get a student visa, can you leave Thailand for a few months and come back and still use it? Is there a time limit for time outside of Thailand before it becomes invalid? - you need to talk that with the institution who would provide you the ED visa. I haven't used it before but I know a lot of people did. I assume that if you leave and don't come back to Thailand for a long time without an excuse, it might not look good and immigration officers can deny your entry if they want.

All your questions are about ED visa so you need to talk with the institution first I assume.

How you so sure you will get covid extension 🤔

Your question doesn't make much sense.

If you are going to go through all the effort to get an ED visa, why are you going to leave the country?

Most ED visas and extension are good for 90 days at a time.

ED visas are issued for active studying in-person inside of Thailand.

If you are leaving the country, you are not actively studying in person inside of Thailand.

If you leave for a few months your visa or extension would have expired already and will no longer be valid by the time you return.

If you are leaving for less than 3 months, you can get a re-entry permit before you leave which will keep the ED visa or extension active until its expiration date, but when you return immigration may be asking you why you are leaving the country while you're studying, especially if it's more than a couple of weeks.