Open 1S. Partner invites you to game with their 3S bid - you only have 14 points after revaluing when partner showed you their fit - 14 + 10 plus points ohhh will you or won't you go to 4S? No. All Pass
Acol Bidding
Open 1S. Partner bids 2D, you rebid your spade suit 2S showing a 5 card suit and partner invites you to game with a 3S bid - you only have 14 points after revaluing when partner showed you their fit - 14 + 10 plus points ohhh will you or won't you go to 4S? No. All Pass
Plan Before You Play
Dummy goes down - count your losers. none in spades, maybe 3 in hearts 2 in clubs and 1 in diamonds. You need to develop another winner to make your contract. The diamonds and clubs appear to be the best suit for this. Leave the heart suit alone until the opposition play it for you, maybe the diamond suit 9 and 10 can produce a winning trick and if we leave the club suit for the opposition to play maybe the JC and 9C will develop into a winning trick.
The Play
A heart lead whew! one problem solved. Win the second lead of hearts then ruff the third heart in dummy. Draw two rounds of trumps, enter your hand with JC and draw out the outstanding trump card. Lead the AC and a low club, (now the 9C is high). win the diamond return and throw the QD on the 9C, the rest of the tricks are yours.
Comment
The decision to go to 4S is made by evaluating the rest of your cards - do you have lots of 9's and 10's which may develop into winning tricks? Or are your small cards exactly that, small cards. In this situation because you are Vulnerable it's probably best not to take the chance - you will be punished harshly if you go light.
The above commentary is intended as a guideline only. As with any Bridge hand there are many ways to bid and play any hand.