Regular or semi-regular 'er' 'o' verbs

Question

I thought on reading this lesson that all 'er' verbs with 'o' in would follow this rule to become 'ue' but then got a test on 'toser' and got that wrong (tried to put tueso, not toso). Plus comer stays as 'o'.

How do we know when to apply this rule, is it a case of just learning the verbs which are semi-regular? Are there a lot or is this pretty doable?

Silvia Piriz · Accepted Answer

Hola Becky W.
Great question! Not all -er verbs with "o" in the stem change to ue. Only certain verbs belong to this group of stem-changing verbs, for example: volver → vuelvo, poder → puedo. Others, like toser or comer, remain regular and do not undergo the vowel change.
There isn’t a rule that lets you predict this just by looking at the verb. It’s more a matter of learning which verbs are stem-changing and which ones are not. The good news is that the number of common o→ue stem-changing verbs is limited and quite manageable, so with practice you’ll quickly get used to recognising them.
Hope this clears it up!
Un saludo
Silvia