The missing words are "have been playing" so the full sentence would be "The boys clothes are very dirty. They have been playing soccer"
It is known as a present perfect progressive verb and is made up as follows:
- The present indicates that it is happening now or very recently
- The Perfect part indicates that the action is completed (or perfected)
- The progressive section shows an ongoing action that at some point has been in progress
The perfect and progressive can be used together as in the example given.
Language is a very complicated thing to learn; there are so many rules to grammar with verbs, adjectives and tenses all being part of the mix. It is certainly worthwhile studying it though, as it is something you will use all through your life.
