"Sometimes" is always one word. "Sometime" can be one word or two, depending on context. You can't use "in" before "sometimes"'. The spelling is "grammar".

sometimes is one word No, you cannot use in before it there is no need for a preposition here

Sometimes, as an adverb, is one word. However, if you are using 'some' to mean 'a few' and 'times' to mean occasions, they are two separate words. For example: 'I sometimes correct people when they spell 'grammar' incorrectly. But there are some times when I don't. On these occasions, I give the person an example of the correct spelling in the hope that they will work it out for themselves.'