1.Please, let me know the difference between: complete and completed.
Example: a complete action / a completed action
2. I know we have to say: "play sports", but I saw the following sentence in a book: "What sports are they practicing". Wouldn't it be: "what sports are they playing???"
completed is either the simple past or the past participle of the verb complete. There is also an adjective complete.
If you don't complete(present tense verb) your test before everyone else has completed(past participle) theirs, then you will look like a complete (adjective meaning total) fool.
Practicing is a type of playing where one tries to improve. This is true whether playing a sport or playing a musical instrument. Practice is a type of learning through playing for a team, or an individual.