Question about mutations in E-conjugation verbs
What does the second sentence of this tip, mean?
"Е-conjugation verbs have mutation in ALL personal forms (if any). It is non-productive behaviour, which in practice means that a lot of popular verb stems still have this behavior (e.g. писать, сказать). However, new Е-conjugation verbs do not get this pattern."
That is, what does "non-productive behavior" refer to?
"Productivity" here is a linguistic term thst basically means that the special conjugations of е-verbs happens in old verbs, but in new verbs, you don't get those stem changes. Check out the Wikipedia article here for a better description of productivity: https://en.m.wikipedia.org/wiki/Productivity_(linguistics)