I shall provide you with information on the usage of ''only'' in sentences. Firstly, you should know that besides being used as an adjective (e.g.: ''He is the only one who survived the fire''), the word ''only'' is, if anything, more commonly used as an adverb to say that something is the one thing that is done, that happens, or that is relevant in a particular situation, in contrast to all the other things that are not done, do not happen, or are not relevant.
1) If 'only' applies to the 'subject' of a sentence, then you put it in front of the subject:
''Only his close friends knew him well''.
''We believe that only a completely different approach will be effective''.
2) If the verb is 'be', you put 'only' after it.
E.g.: ''There is only one thing we should do''.
3) If the verb is not 'be' and 'only' does not apply to the subject, you usually put it in front of the verb or after the first auxiliary verb, regardless of what it applies to. For example, instead of saying 'I see my brother only at weekends', you usually say 'I only see my brother at weekends'.
4) When used for emphasis (if you wish to be rather clear or emphatic about something), you have two options: you may put 'only' either immediately in front of the word/clause to which it applies or after it; the latter is used for extra emphasis.
E.g.: ''He played only classical music''.
E.g.: ''We insisted on being interviewed by women journalists only.
5) ''Only'' and ''Just'': Sometimes they can be used interchangeably (with the meaning of ''no more than...''), but not always.
E.g.: ''I only have three kids.'' or ''I just have three kids.'' (both words work in this case)
Now, have a look at these examples:
E.g.: ''I have just fixed your car''. (You car has been fixed by me a few moments/minutes ago)
E.g.: ''I have only fixed your car''. (no other cars have been fixed by me, other than yours!)
Can you tell the difference now?
Hope you have understood it! :)