RADICAL CHANGING VERBS; PRESENT TENSE

Radical Changing Verbs (also called Stem Changing Verbs) are verbs, mostly in the Present Tense, where the internal vowel changes when we conjugate. 

To put it simply, we have the verb Dormir (to sleep) - this is the infinitive. 

To use it in the present tense we need to change it to 
yo duermo
tú duermes etc....

The "o" in the infinitive changes to "ue" when we use it in the present. 

We have three different kinds of these verbs, representing three different changes....
o - ue          (dormir, poder....)

e - ie           (pensar, entender...)

e - i              (pedir....)

Watch the video, do some practice and, if you want, do some extra practice from the link below.....

There are more practice exercises here.