different usage of the word "only"

Solution 1:

Only the trees were somewhat damaged by last year's storm.

Meaning: Nothing except the trees was somewhat damaged by last year's storm.

The only trees were somewhat damaged by last year's storm.

Meaning: The few trees which existed were somewhat damaged by last year's storm.

The trees were only somewhat damaged by last year's storm.

Meaning: The trees were not completely damaged by last year's storm.

The trees were somewhat damaged only by last year's storm.

Meaning: The trees were somewhat damaged by nothing except last year's storm.

The trees were somewhat damaged by last year's only storm.

Meaning: The trees were somewhat damaged by the one storm which occurred last year.

Source

Solution 2:

To the point, your sentences 2 and 3 actually have exactly the same meaning. Although the scope of the quantifier "only" usually changes based on what follows it in the sentence, in the exceptional usage of sentence 3 it is modifying only the preceding word, "English".

He speaks only English. == He speaks English only.

Solution 3:

He only speaks English.

He does not read or write it. However, a listener may take this to mean the same as the next sentence.

He speaks only English.

He does not speak any other languages.

He speaks English only.

Same as above.