Don't read this until you have written an answer on your own.

There are several options.  The key is in the first premise.  Here are several of the possibilities for making this argument valid.

All teachers who come to class on time are good teachers.  Professor Simpson comes to class on time.  He must be a good teacher.

Only teachers who come to class on time are good teachers.  Professor Simpson comes to class on time.  He must be a good teacher.

If Professor Simpson comes to class on time, then he is a good teacher.  Professor Simpson comes to class on time.  Therefore, he is a good teacher.


Don't read this until you have written an answer on your own.

Everytime there is someone in the front yard, Spot bark.  Spot is barking, so someone must be in the front yard.

This argument is invalid.  The first premise does not exclude the possibility that Spot barks at other times as well, when another dog or a cat is in the front yard, or when he is hungry or thirsty, or when he needs to go outside.    Hence, even if premise one and two are true, it IS possible for the conclusion, "Someone must be in the front yard," is false.

To make this a valid argument, you would have to rewrite the first premise.

The only time Spot barks is when someone is in the front yard....

Everytime that Spot barks someone is in the front yard...