They have a baby sitter, so they can go out very often.

→If They didn't have a baby sitter,  they couldn't go out very often.

They have a baby sitter, so they can go out very often.

→If they didn't have a baby sitter, they couldn't go out very often.

d.incorporate=include: bao gồm, kết hợp
d.significant difference=striking contrast: sự khác biệt rõ ràng