With someone, alone, or together
Say 友達と勉強します and 一人で勉強します. Point to two people for the first sentence and one for the second; only then choose the particle.
1. Person + と names a companion
In 友達と公園へ行きます, the person comes directly before と. That block answers ‘with whom?’; 公園へ separately marks the destination. The verb states what the speaker does with that companion.
Do not use で for a specific companion. 友達で行きます does not mark the relationship as ‘with a friend’. The same pattern works with 彼と or 彼女と. Their possible meaning as a partner depends on context; here they are neutral practice people.
友達と公園へ行きます。
彼と勉強します。
2. 誰と asks with whom
Replace the known person in the companion block with 誰 and keep と: 先生と話します becomes 誰と話しますか. The answer puts a specific person in the same position: 友達と話します.
A natural answer can also be 一人で行きます. This does not name a companion but says the action happens alone. 誰で行きますか uses で where the question asks for a person with と.
誰と勉強しますか。
友達と勉強します。
3. 一人で, みんなで, and 一緒に
一人で is the fixed form for ‘alone/by oneself’. みんなで says a group acts as a whole. In both forms, で describes the manner or group arrangement, not a location. Therefore, do not automatically assign a location meaning to every で.
一緒に emphasises acting together. It can stand alone when the participants are clear, or strengthen a companion: 友達と一緒に勉強します. 一人と is not correct for ‘alone’; it sounds as though ‘one person’ is a companion.
4. Keep relationships and counts distinct
In パンと卵を食べます, と joins two things in a list. In 友達と食べます, と links a person as companion to the action. In 学校で食べます, school is the place; in みんなで食べます, the group acts as a whole. Look at the word before the particle each time.
The unit also counts people. 一人 is ひとり, 二人 is ふたり, and from 三人 onward you usually use a number + にん, as in 三人, さんにん. Counting states how many people there are; 一人で additionally says someone performs an action alone.
学生は三人です。
クラスのみんなで勉強します。
5. Track who takes part
Read the existing weekend dialogue. Speaker B first goes with a friend, then eats as a group, and studies alone on Sunday. Mark 友達と, みんなで, or 一人で in each answer before translating the action.
Then create three new fictional activities with the same three relationships. In the final check, the prompt states whether a friend, a group, or nobody accompanies the action.