Surely a player of your bridge skills is familiar with most or all of the following tips you are about to read. But is your partner?
When giving partner a ruff, the card you lead is suit preference.
The return of a relatively high spot card asks for a return in the higher ranking of the two remaining suits. A return of your lowest card asks for a return in the lower ranking of the two remaining suits. The return of a middle card is designed to drive partner crazy. It actually means you have no preference.
The ACBL Board of Directors removed a legal gray area surrounding semi-balanced notrump openers when it approved a change to the definition of a notrump opening in the General Convention Chart. The change allows players to open 1NT with hands that include a singleton ace, king or queen, as long as they do not also contain a doubleton.
After a 1♦ overcall, a major suit response can be made with only 4. But, after a 1♥ overcall, a 1♠ response guarantees at least 5. Two level responses guarantee at least 5.
Conclusion: If they overcall anything other than 1♦, then responder's bid of a major guarantees 5 or more. If they overcall 1♦, then responder is allowed to bid 1♥ or 1♠ with only 4.
Warning: This needs to be studied, studied again and memorized. It isn't complicated, but causes complication if it isn't digested properly.