A TIP FOR JUNIORS
You are dealt this hand: ♠K3  ♥KJT863  ♦92 ♣J53

Aha, a perfect hand for opening a weak 2H. .... but you are not the dealer. Partner is dealer and opens 1S.
First, assume no overcall from the next person to bid. Now you: You want to bid 2H but you can’t because that promises at least 10 hcp.
Your best bid is 1NT, followed by a second bid of 2H. This sequence shows exactly this sort of hand.
DO NOT assume that you won’t get another bid. Most of the time you will.
Let’s say your partner has: ♠AJ876 ♥Q2 ♦KQT3 ♣Q8. She won’t want to play in 1NT with two doubletons so she is going to bid 2D. Then, when she hears you bid 2H she gets the picture and, as she has only minimum hand values, she will pass.
You will play in the optimal contract of 2H (probably making 3).
Second, assume an overcall of 2C follows your partner’s opening 1S. Your best bid is now X, followed by a second bid of 2H on the next round. The double, after an overcall of your partner’s opening bid, is an extremely valuable tool. My recommendation is that you should use it to mean: “ Partner, I have a hand worth bidding (at least 7 hcp) but I can’t show you my best suit because I’m not strong enough to bid at the two level.”
Again, partner will make a rebid of 2D, allowing you to bid 2H.