When to support 1♥/♠ to 2♥/♠ and when to respond 1NT

The simple way to play is that a raise of 1♥/♠ to 2♥/♠ always shows four+ cards, just like 1♥/♠-3♥/♠. The trouble is, that’s far too restricting.

When partner opens 1♥/1♠, she will have five+ cards more often than not. She will be far happier hearing you support her, than responding the nebulous 1NT. So support as often as you can.

Partner has opened 1♠. With which of these would you respond 2♠ and with which 1NT?:
 

Answers:

(a). Raise to 2♠. If partner now bids 2NT, she is showing 17-18 with (normally) just four spades. So raise to 3NT.

(b). Raise to 2♠. Although your spades are just three low, responding 1NT is misguided. Although you have not shown a balanced hand, partner will nonetheless pass with 15-16 balanced; declaring 1NT will be nerve-wracking affair given your hearts (or lack thereof). Note that responding 2♦ would be incorrect, as you fail the Rule of 14 (points in the hand + number of diamonds do not get to 14).

(c). Respond 1NT. You exceptionally prefer 1NT to 2♠ with three spades, because your spades are low and you are very flat. 

The guideline is to raise to 2♥/♠ with three cards in preference to responding the dustbin 1NT, unless you have both a very flat hand (eg 4333) and three small cards.