How are ‘stem-changing’ verbs conjugated in Spanish?

In all languages, verbs follow certain grammatical rules. In Spanish, all verbs are made up of a stem (which tells you what action is being done) and an ending (which specifies who is doing the action and the tense it is being done in). In regular verbs in Spanish, the stem stays the same, while the ending changes. For example, in the verb ‘hablar’, which is regular, ‘habl’ forms the stem, and ‘ar’ the ending, so the present tense conjugations are as follows - ‘habl- o’, ‘habl- as’, ‘habl- a’, ‘habl- amos’, ‘habl- áis’, ‘habl- an’. Nevertheless, there are many irregular verbs in Spanish, some of which can be classed as ‘stem-changing verbs’. This means that they conjugate the ending in the same way as regular verbs, however the stem changes in the yo, tú, él/ella/usted and ellos/ellas/ustedes forms. The stems tend to change in the following way: e -> ie (e.g. empezar: empiez- o), o -> ue (e.g. costar: cuest- o), e -> i (e.g. repetir: repit- o), i -> ie (adquirir: adquier- o), u -> ue (jugar: jueg- o). To demonstrate fully how the verb would conjugate in the present tense: empiez- o, empiez- as, empiez- a, empez- amos, empez- áis, empez- an.