The Rule of 14
You may respond in a new suit at the two-level when your HCP + the number of cards in your suit equals fourteen or more.
If not - but with six + points - respond 1 NT.
The Rule of 15
In the fourth seat after three passes -
If you too have less than an openoing hand you should open the bidding if the number of high card points and the number of cards in your ♠ suit equals fifteen.
Otherwise the opposition may find a fit in their suit by overcalling.