Can you play a land on your turn if you've already played one using a rule that gives you an additional land drop?

By default, you may play one land on your turn during your main phase while the stack is empty (CR 305.1). This is often called your 'normal' or 'base' land drop for the turn.

Certain cards and effects — such as Explore, Oracle of Mul Daya, or Fastbond — grant you the ability to play one or more additional lands during your turn (CR 305.2). Each such effect independently adds to how many lands you may play that turn; they all stack together.

So if you have already used your normal land drop and then resolve an effect that says 'you may play an additional land this turn,' you immediately become eligible to play another land — and you may do so as long as you're in a main phase with an empty stack (CR 305.1, 305.2).

Concrete example: You play a Forest (using your base land drop). You then cast Explore, which says 'you may play an additional land this turn.' You may now play one more land from your hand that same turn. If a second Explore resolves afterward, you could play yet another land.

Important caveat: if an effect says 'you may play a land' (not 'an additional land'), it typically creates a separate permission that only applies if you haven't used your normal land drop yet — wording matters. Always read each card's exact text in light of CR 305.2.