exercise

I go to the gym and exercise.

To 'exercise' is to move your body in a way that makes you stronger.  This can mean lifting weights or doing something like running that makes your heart beat faster. Notice the following:

1. What do you do for exercise?
   
2. Exercise is a type of medicine.
   

have to

I exercise three times a week because I have to.

When we 'have to' do something it is an obligation.  We don't have a choice or an option. Notice the following:

1. I have to read about 50 pages tonight.
   
2. I have to get 93 on my exam to pass the class with an A.
   

lose weight

I weighed too much, so I decided to lose weight.

When we decrease how much our body weighs we are 'losing weight.'  We can do this by exercising more or changing what or how much we eat.  Notice the following:

1. She wants to lose weight before her wedding.
   
2. The doctor told him to lose weight.
   

if possible

I decided to lose weight, if possible.

We can add the phrase 'if possible' to a statement if we don't know that it is a real possibility. Notice the following:

1. If possible I would like to exercise every day.
   
2. Please try to be early if possible.
   

weakness

Chocolate is my weakness.

Your 'weakness' is what you can't say no to or resist.  In this case if there is chocolate you will eat it.  Notice the following:

1. She knows all his weaknesses.
   
2. He has a real weakness for potato chips.