Present Simple versus Present Continuous

To start with I usually called present simple as habitual present because it shows better when we use this tense. It's referred to habits, routines, general truth or similar statements, while present continuous is referred to a particular moment of an action or something that happens in a particular time or period of time.
Have a look these examples:
   What do you do?  ---  I'm a teacher; I teach English at school
   What are you doing this week? -_- I'm explaining present tenses to my students
In many occasions, habitual present is used with frequency adverbs such as usually, often, hardly ever, sometimes, etc.
If you want to practise with these tenses you can do these exercises:

• exercise 1: a good explanation about both tenses with exercises at the end
• exercise 2: another longer explanation but it's useful for the examples and it includes many special cases such as have, think, want, etc. Very few exercises but interesting.
• exercise 3: short simple explanation and some exercises to practise
• exercise 4: a filling the blanks exercise
• exercise 5: a very complete page with grammar and exercises at the end.

See you in class!