[RadioKid]

These kind of question is difficult for me because I do not know the grammatical word enough in English.

I mean I can explain it in Japanese. It is "動詞の活用" issue.　

"食べます" has "食べ" as "動詞：食べるの活用形(連用形)" and "ます”.

"します" has "し" as "動詞：するの活用形(連用形)" and "ます".

The difference is .....

"食べる" has 下二段活用.
"する" has サ行変格活用.

つまり、「動詞の活用形が違う」ということなんです。

My answer is "verb-variation-types are different".

However, I have no vocabulary to explain these explanations in English.