I only have or I have only? ////I will only be or I will be only?
Normally you use adverbs after the auxiliary and before the main verb, so how can I use when an auxiliary verb is used as a main verb?

I have only 10 apples in this basket.
I only have 10 apples in this basket.

Normally you use an adverb after the verb "to be", so how can I use in this example with the auxiliary "will"?

I will only be a teacher.
I will be only a teacher.

Thank you in advance

Question

I only have or I have only? ////I will only be or I will be only?
Normally you use adverbs after the auxiliary and before the main verb, so how can I use when an auxiliary verb is used as a main verb?

I have only 10 apples in this basket.
I only have 10 apples in this basket.

Normally you use an adverb after the verb "to be", so how can I use in this example with the auxiliary "will"?

I will only be a teacher.
I will be only a teacher.

Thank you in advance

Answer

This is a "squinting" modifier. This means placement can affect meaning. In your examples, "I only have 10 apples in this basket", the meaning is "I alone" have ten apples. By the way, "have" is not an auxiliary here, but the main verb. In the case of "I have only 10 apples", then you have merely ten. In the second set of examples, I think the meaning is clear either way, but "I will be only a teacher" is better.

Answer

It's pretty simple: "only" modifies whatever immediately follows it.  This is a rule for the use of "only", not for adverbs in general.

Answer

I only have 10 apples in this basket. 
I will only be a teacher.

"I have only 10 apples in this basket. " sounds very old-fashioned so you might come across it in some old stories. Sorry I'm not sure about the grammar of this.