My dear and respected teachers,
1: "His wife had two babies at the first time OR for the first time."?
2: "I even can buy an old model of a bike for... OR I even can buy an old model of bike for..." ?
3: "Users will save their credits in their main IDs." correct?
1) FOR the first time. Assuming she's having twins!
2) of A bike -- if you are just saying you are buying a bike (without special emphasis)
of bike -- if you really want to emphasise it is an old model
3) Do you mean credits are electronically stored on the ID or that users will save money by having an ID? For the former, "in" and "on" are both OK, for the latter, only "on" is OK.