Learn Hardcore Japanese: watasi to tomodati ha tugi no keikaku nituite hanasiaimasu. - My friend and I will discuss the next plan.

と is the particle that connects two or more items in an exhaustive list, so 私と友達 means exactly “my friend and I.” や would imply a non-exhaustive list (“my friend, I, and others”), which isn’t intended here.

Could we say 友達と私は instead of 私と友達は? Which is more polite?

In Japanese etiquette it’s generally more polite to mention the other person before yourself, so 友達と私 is slightly more modest than 私と友達. Both forms are grammatically correct, but putting 友達 first can sound more considerate in formal contexts.

Why is は placed after 私と友達? How does it work with a compound phrase?

The particle は marks the topic of the sentence, and a topic can be an entire phrase. Here 私と友達 together form one unit (“as for my friend and me”), so you attach は to the end of that phrase.

Could we use が instead of は in 私と友達が…? What difference would that make?

が marks the grammatical subject, often for new or emphasized information. は marks the topic or contrast. If you said 私と友達が次の計画について話し合います, it emphasizes “it is we who will discuss (not someone else).” Using は simply states “as for us, we will discuss …” without extra emphasis.

What does 次の計画 mean? Why is の used between 次 and 計画?

The particle の here links 次 (“next”) to 計画 (“plan”), forming the noun phrase “next plan.” It’s like saying “plan of next,” which in English we invert to “next plan.”

What is the function of について? Could we just say に?

について is a compound particle meaning “about” or “concerning.” It marks the subject matter of discussion. に alone would mark a direction or indirect object (“to” or “at”) and wouldn’t express “about.” Saying 次の計画に話し合います would be ungrammatical in this context.

What does 話し合います mean? How is it different from 話します?

話し合う is formed by the verb stem 話し- (“speak”) plus 合う (“mutually”), so it means “to discuss” or “to talk over (with each other).” 話します simply means “to speak” or “to talk,” without necessarily implying a two-way discussion.

Why is the verb in the –ます form, and is it present or future tense?

The –ます form is the polite non-past form in Japanese. Non-past covers both present and future depending on context. Here, since you’re talking about a planned action, 話し合います is best translated as “(we) will discuss.”

Do we need to include 私 in the sentence? Couldn’t we just say 友達と次の計画について話し合います?

Japanese often drops subjects when they’re clear. 友達と次の計画について話し合います implies “I will discuss … with my friend.” Including 私と友達 explicitly states that both you and your friend are the ones doing the discussing. It’s not mandatory but clarifies who’s involved.