に for direction / existence, で for action
Japanese uses two different particles to mark a place, depending on what's happening there. に marks a destination ('to a place') or where something exists ('at a place'). で marks where an action is performed ('in / at a place'). They're not interchangeable. Today we nail down the contrast — and pick up も ('also') and a clearer view of が while we're here.
に — going there
When the verb is one of motion (いきます go, きます come, かえります return), に marks the destination. がっこう に いきます。 = 'I go to school.' にほん に きます。 = '(I) come to Japan.' うち に かえります。 = 'I return home.' Think of に as the arrowhead of an arrow pointing at the place. The casual swap へ ('e' as a particle) means almost the same thing — you'll see both in the wild.
に — being there (existence)
に also marks where something or someone exists, with あります (for things) and います (for people / animals). こうえん に ねこ が います。 = 'There's a cat in the park.' つくえ の うえ に ほん が あります。 = 'There's a book on the desk.' Notice the order: [place] に [thing] が います/あります. The thing exists, and に says where.
で — doing there
When the verb is an action verb (eat, drink, study, work, watch, read), で marks the place where it happens. レストラン で たべます。 = 'I eat at a restaurant.' がっこう で べんきょう します。 = 'I study at school.' うち で テレビ を みます。 = 'I watch TV at home.' If you'd say 'in / at' for the location of an action in English, it's で, not に.
The contrast that gets people
Same place, different particle: がっこう に いきます。 ('I go to school' — destination) vs がっこう で べんきょう します。 ('I study at school' — action). レストラン に いきます。 ('I go to the restaurant') vs レストラン で たべます。 ('I eat at the restaurant'). The verb tells you which one. If unsure, ask: is the place where the action happens, or just where you're heading?
も — 'also', replacing は or を
も means 'also / too' and replaces は or を (you don't say は も). 私 は がくせい です。アナさん も がくせい です。 = 'I'm a student. Anna is a student too.' Note アナさん も, not アナさん は も. To say 'I read this book too', the を disappears: この ほん も よみます.
が — new info / existence
が has two main A1 jobs. (1) It marks the existing thing in あります/います: ねこ が います. (2) It marks new information — the answer to a 'who/what?' question. だれ が きます か。 ('who's coming?') → やまだ さん が きます。 ('Yamada is coming.'). The thing answering the question takes が, not は, because it's the new news.
