Learn Hardcore Japanese: kinou no hirugohan ha douryou to warikan ni simasita. - I split the bill for yesterday’s lunch with my coworker.

In Japanese, は marks the topic: “As for yesterday’s lunch …” It sets 昼ご飯 as the topic to comment on. Using が would mark it as the subject, not highlighting it as the topic or point of contrast.

Why is と used after 同僚?

The particle と means “with” when used after a person. 同僚と means “together with a colleague.” It shows partnership in the action.

What is the structure 割り勘にしました, and why not just 割り勘しました?

割り勘 (“splitting the bill”) is a noun, and にする means “to decide on [something].”
So 割り勘にしました = “(We) decided to split the bill.”
Saying 割り勘しました focuses on the fact “We split the bill,” whereas 割り勘にしました emphasizes the decision process.

Why doesn’t this sentence have an explicit subject?

Japanese often omits the subject when it’s clear from context. Here, it’s understood that “I” or “we” (the speaker and colleagues) is the subject, so it isn’t stated.

Can I say お昼ご飯 instead of 昼ご飯, and what’s the difference from 昼食?

Yes. お昼ご飯 adds the honorific お, making it a bit more polite or friendly. 昼食 is more formal or written, while 昼ご飯/お昼ご飯 is common in everyday speech.

Why is the time expression 昨日の昼ご飯は placed at the beginning?

Japanese often places time, place, or topic phrases at the front to set the scene. Starting with 昨日の昼ご飯は tells the listener when and what you’re talking about before moving on to the action.

Could I drop に and say 同僚と割り勘しました?

Yes, you can. 同僚と割り勘しました simply states “I/We split the bill with a colleague.”
Using 割り勘にしました instead highlights the decision to do so.