What are radical-changing verbs?

Radical-changing verbs, or stem-changing verbs, are common in Spanish. Whilst they are not regular verbs, they do follow a pattern, making them easier to learn than verbs that are fully irregular. It is important to remember that the 'nosotros' and 'vosotros' forms do not change, but instead keep the infinitive stem. For example, in the present tense, you conjugate 'pensar' as: (yo) pienso, (tú) piensas, (él/ella) piensa, (nosotros) pensamos, (vosotros) pensáis, (ellos/ellas) piensan. 'Pensar' follows one of the most common changes, which is 'e' to 'ie'. More verbs following this form are 'despertarse', 'empezar' and 'querer'. Other changes are: 'o' to 'ue', 'e' to 'i', 'i' to 'ie' and 'u' to 'ue'. The most common verb in the 'u' to 'ue' group is 'jugar': (yo) juego, (tú) juegas, (él/ella) juega, (nosotros) jugamos, (vosotros) jugáis, (ellos/ellas) juegan. As you can see, the verb endings are the same as those of regular verbs; it is only the stem that changes. You will soon recognise the form and conjugate radical-changing verbs correctly every time you use them!