199. Stop talking or you won't understand the leson.
-> If you stop talking, you will understand the lesson.
-> If you don't stop talking, you won't understand the lesson.
120. l don't know her number, so l don't ring her up.
-> lf I knew her number, I would ring her up.
121. l don't know the answer, so l can't tell you.
-> lf I knew the answer, I could tell you.
* If 1 : If + S1 + Vs/es, S2 + will + Vinf
* If 2 : If + S1 + V2/ed, S2 + would + Vinf