These verbs are known as stem-changing or radical-changing verbs, because what makes them irregular is that only the stem (the part of the verb that comes before -ar, -er or -ir) changes.
For example, any form of the regular verb hablar, whatever the tense or mood, starts with habl-. But the verb sentar, which usually means "to sit," can have its stem change. Fortunately, the stem of verbs like sentar change in a predictable way, and that is that the e changes to ie whenever it is stressed. When them stem isn't stressed, it remains unchanged.
Thus, the singular present indicative forms all change: siento (I sit), sientas (you sit), sienta (he/she/you sit). But the first- and second-person plural forms do not: sentamos (we sit), sentáis (you sit). The third-person plural form changes: sientan (they sit).
The same pattern — the i changing to ie when stressed — follows for all other forms. You can see how this works by looking at the complete conjugation of pensar.
Other verbs that are conjugated in exactly the same way are acertar, alentar, apretar, arrendar, atravesar, calentar, cerrar, confesar, despertar, encerrar, enterrar, gobernar, helar, manifestar, merendar, recomendar, reventar and sembrar.
A similar pattern can be seen with verbs such as sentir. The stem of sentir changes in the same way as do the stems of sentar and pensar, but the verb has some differences in conjugation because it is an -ir verb rather than an -ar verb. Verbs that are conjugated in the same was as sentir include adherir, advertir, arrepentir, asentir, concernir, conferir, convertir, deferir, desmentir, digerir, disentir, divertir, herir, hervir, inferir, invertir, mentir, pervertir, presentir, preferir, resentir and referir. The same goes for entender, which, except for the stem, follows the pattern of other -er verbs.
There are a few verbs that have the e-to-ie stem change but are also irregular in other ways. For example, querer is conjugated in much the same way as entender, except querer sometimes has the r of its stem change to rr.
