According to standard American English grammar, "but" is not an acceptable word to start a sentence with - it is a conjunction to be used in the middle of a sentence. However, in sloppy American useage, many people do start sentences with "but" when speaking. In general, you should avoid starting sentences with "but" unless there is a specific reason (such as colloquial dialogue).
Alternative words for "but" at the start of the sentence include "however", "alternatively", "conversely" and "nevertheless". These are adverbs and are acceptable beginning words.
Yes, the sentence "your retina is healthy" is correct.
Of course you can.
It depends how you use it. If you use it after something it can be correct. But being in a sentence by itself isn't correct.
Of course. Make sure you use the correct form for what you are talking about: your = someone's something; you're = you are
Use is present. Used is past. The correct sentence is, This is used for....
If you use 'you and he' as the subject of the sentence, it is correct: You and he will meet when we get to the restaurant.
To start with ME, not myself.
A correct answer would be yes.
He's bit the apple. This is a correct sentence (don't forget to start a sentence with a capitol letter).
Your sentence is structurally correct.
The correct sentence form is: Abby whispered, "The movie is about to start."
The correct punctuation for the sentence "In September you and him get to start going to middle school" would be: "In September, you and he get to start going to middle school."