is a 接続助詞（せつぞくじょし） (conjunction particle). This means that it is used for linking the (A) and (B) parts of a sentence, and describing that link in some way. The ‘description’ that し
will highlight is ‘reasons’. Regularly it is translated simply as ‘and’.
may be connected to the end of almost any word, but will require a だ
before it when it comes after nouns or な-Adjectives
will follow every ‘reason’ that is being listed in a sentence. However, the last し
may be replaced with から
, before the ‘result’ is stated.
She can play the piano and she can do sports. I think there isn’t anything she can’t do.
This restaurant is expensive and doesn’t taste good. There are no good parts.
He is serious and nice. I am glad that he is my friend.
Today is my day off, and it’s sunny, so let’s go to the park.
will indicate that each ‘reason’ being listed is of equal weight/importance. Because of this, it would not be appropriate to use in sentences where one of the reasons clearly far outweighs everything else.
It was raining yesterday and my grandmother also passed away, so I’m sad. (Unnatural Japanese, as the level of sadness between losing your grandmother, and it being a rainy day would be completely different)
In all of its uses, し indicates things that are 矛盾（むじゅん）無（な）く共存（きょうぞん）する, which basically means ‘things that coexist, without contradicting each other’. This is the primary reason why each thing listed must be of equal importance (in terms of the overall story that the speaker is trying to convey).