I am just a beginner so this question may seem silly.

When you play a D7 chord (F#-C-D) say on a base clef, then do I need to play F# instead of an F every time I encounter an F until the natural sign appears?
Or does this "once sharped, always sharped" rule applies only when you play a single note and not the whole chord?

If the chord is "written out" in notes on the staff the sharp only applies to the particular measure it's in, and goes back to being natural in the next measure with or without a natural sign. If there *is* a natural sign in the next measure on an F it's called a "courtesy" natural - just a friendly reminder that it's now Fnat. But it would be Fnat anyway.

If you're playing a D7 from a chord chart for a lead sheet it doesn't effect anything except that particular chord, although I suppose it's not likely to have an Fnat in the melody line at the same time.

Cathy

Your question shows that you're a little confused on the topic of key signatures and accidentals (sharps, flats, and naturals). Something is only "always sharped" if there's a sharp for a respective tone in the key signature (like how the key signature for G Major is one F Sharp). As jotur said, if you see a sharp added to a note mid-measure, then that pitch (and only that pitch - not the same pitch up or down and octave, or two, etc.) will remain sharped should the same note be encountered again, but only for the remainder of that one measure.

There`ll be situations particularly on the bass clef where you can leave out the Fsharp altogether so things don`t sound so clarty. Or one of the other notes maybe. It`s not as noticeable in the higher octaves. In my opinion.