stem-changing e to ie verbs

Question

The lesson says when the "e" is stressed you change the e to ie. This seems so vague to me since it does not follow any pattern as in the example below:

acercar=to come closer no change to ie

acertar=to hit upon yes, it changes to ie

The e is stressed in both cases so why does one verb change and not the other?

Answer 1

The lesson says when the "e" is stressed you change the e to ie. This seems so vague to me since it does not follow any pattern as in the example below:

That rule only apply to those irregular verbs that follow that pattern, like "acertar". However, "acercar" is regular, like "hablar", so it does not follow that pattern. Remember that irregular means 100% unpredictable, which means that memorizing it is the only thing you can do.

Regarding "acertar", let me give you the (wrong) regular pattern with the stressed vowel in bold, so you can apply the rule:

acerto

acertas

acerta

acertamos

acertáis

acertan

The reason why one changes, but not the other? When these e -> ie changes happened, the verb "acercar" had not been created yet, so it never underwent this transformation. The same happened in English with words borrowed or created before and after critical sound changes.

Answer 2

Well, then you really just have to learn which verbs stem-change and which don't. Eventually you'll get a good enough feel as to whether a verb stem-changes or not.