Sure! Here are the completed sentences using "if" or "when" and the correct forms of the verbs in brackets:
If you speak slowly, I can understand you.
When the weather is nice, we always go to the beach.
If I am late, my parents get angry with me.
When I arrive home from work, I usually do sports, and you have a cup of coffee.
If you study hard, you get good marks and you are always in a good shape.
If I feel tired, I go to bed early.
If students do not do their homework, the teacher punishes them.