Why there is an A major chord in a song with a key F# major?

[Nepal]

i am new to music theory, and i got the basic concept of the chords that can be formed from the F# major scale. But there is no A major note in F# major scale. But when i looked in one of the song, its chord progression is F# / A major / E major / B major... can you explain the theory behind it plz...

 

[Pumpthrust]

i am new to music theory, and i got the basic concept of the chords that can be formed from the F# major scale. But there is no A major note in F# major scale. But when i looked in one of the song, its chord progression is F# / A major / E major / B major... can you explain the theory behind it plz...

1. I see an F# with no qualifier. Is that an F# major or F# minor chord? Because, normally I would read it as a major chord, but since you have indicated all the other chords to be major, do you mean that F# chord is minor?

Because, if it is indeed an F# min chord, the progression is not in the key of F#, (I see the key of E major) based on the chords you presented. Learn how chords are constructed in every key from the corresponding scale, learn your key signatures, and this will make sense.
F# A C#...F minor triad
A C# E...A major triad
E G# B...E major triad
B D# F#...B major triad
....All those chords belong to the key of E major. (Assuming you meant F# to be a minor chord in that particular progression).

 

[Nepal]

it is F# major / A major / E major / B major