Sometimes I want to add emphasis to the frequency of something, so I say 很经常 (instead of just 经常), but my teacher has pointed out it's wrong. (Judging from Google, it seems I'm not the only one making this mistake; see e.g. Sohu news, NPC.gov.cn, or the World of Warcraft forum where 很经常 is used.)
However, I'm not precisely sure why 很经常 is wrong: if I'm saying 很经常 wrong, it's likely I'm making similar 很[word] mistakes, so it would be beneficial to identify these too. Sometimes we can add 很 before a word, and other times we can't; e.g. looking up some example sentences on YouDao, we find:
I'm not sure what the rule/pattern is here.
Question: Why is 很经常 ("very often") wrong?