I know there's a past topic with this question already, but honestly I
couldn't understand the explanation very well.

Thank you very much for the input and time.
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word-choice words nuances verbs
Difference between 増やす and 増加する?
The meanings are mostly identical. The one main difference is that 増{ふ}やす is
explicitly 他動詞{たどうし} (transitive), whereas 増加する could be either 他動詞{たどうし}
(transitive) or 自動詞{じどうし} (intransitive) depending on context. The
intransitive version of 増{ふ}やす is 増{ふ}える.

Past there, there are some differences in connotations and register, which are
inherent to the distinction between _kun'yomi_ words and _on'yomi_ words.
There was a recent post about this subject that you might find informative:

_[Difference between onyomi words and kunyomi
words](https://japanese.stackexchange.com/questions/32582/)_
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First, I must say they have grammatical difference. 増やす is a _transitive_ verb
while 増加する is _intransitive_. Dictionaries may say it's transitive too, but
I'm fully confident that the usage is already out of use. So synonyms are 増やす
/ 増加させる and 増える / 増加する.

In meaning, they both mean increase in numeral value/amount, but have
difference in (aside from formality) that when used with objects, 増加する treats
them as if they're collective, thus doesn't match with something you count one
by one.

> ○ 固定客が増加する  
>  ?? 常連客が増加する  
>  ○ 常連客が増える  
>  ○ 常連客の数が増加する

In these examples, 固定客 and 常連客 both mean "steady customer", but the latter
typically indicates those go so frequently that become some sort of
acquaintances, in this case it's very difficult to use 増加.
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answer