If you play the notes that comprise a D major chord: D, F# and A (the root, the third and fifth respectively) and raise the F# one half step to G, you have created a suspended fourth. It’s a pretty sounding chord and one you’ll surely want to use in at least a few of your songs. Note that all F#s voiced in the major chord would need to be raised to the G note.
The raised third note sounds like it wants to resolve back to the third so the easiest and most common way to employ it is by playing the suspended chord followed by the major (Dsus4 to D) or vice versa (D to Dsus4). You could do this movement once or several times.
So the formula is: root, 4th, 5th of the major scale. Example: To create an A suspended 4th use the A major scale as the basis which is A, B, C#, D, E, F#, G#, A octave. The root is A, the 4th note is D and the 5th is E so A, D, E are the notes of an Asus4.
There are other uses that will be covered in future posts.