As an adjective, we use only in front of a noun or one, or before another adjective or a number:

Is that your only copy of the book?

He was the only one who could read in the village.

Not: He was the only who could read …

That was the only large t-shirt left in that colour.

There were only four United fans in the room.

We can use only as an adverb in different positions, depending on its focus. If the subject is the focus, we put only in front position:

Only Jason knows where the key is kept.

Only a very small bed will fit in this room.

If the focus is on another part of the sentence, we usually put only in the normal mid position for adverbs (between the subject and the main verb, or after the modal verb or first auxiliary verb, or after be as a main verb):

I only go home once a month. (between subject and main verb)

She had only arrived at midnight the night before. (after the first auxiliary verb)

She’s only sixteen. (after be as a main verb)

If the focus is a whole clause, we can put only in front position:

My arm hurts but only when I try to raise it.

See also:

Not only … but also

If only