اذا

If I am at home in the evening, I lock the door with the key.

My friend can walk to the park if the weather is nice.

If the food in the refrigerator is old, I do not eat it.

If you did not understand the question, ask the professor after the lesson.

If you do not find help in the office, call me.

If the program is difficult, there is another easy program on my phone.

If I do not understand the program, I need help.