Why cannot I use ^I did hear^? It is quite common among English speakers to speak like this
"I did hear…" sounds like it has a lot of emphasis, especially if someone asks for clarification. I don't know how you do the same in Polish, but I think it is too distinct.
Possible UK English context (a collapsing alibi?):
– I heard your voice in the shop.
– You can't possibly have heard me there! I never go to that shop on Mondays!
– But I did hear your voice in that shop on Monday!
"w sklepie" is pretty pronouncable. Especially that what you in fact pronounce is rather F than V. Although when you look at the letters, it seems pretty clustered.
Generally "we" is used before a cluster of two consonants, the first of them being W or F. But there are some other situations, like "we mnie", for example.