Learn Hardcore Japanese: ryousin ha syuumatu ni ryokou wo simasu.

は is the topic marker. It indicates that 両親 (“my parents”) is what the sentence is about. You can think of it as “As for my parents…,” rather than marking them as the grammatical subject.

Why is 週末 followed by に? What does に mark here?

に marks a specific point in time when an action occurs. Here, 週末に means “on the weekend.” Without に, it would just be “weekend” without clearly indicating “when.”

Why is 旅行 followed by を and then します? Can’t we just say 旅行します?

旅行する is a compound verb formed from the noun 旅行 (“trip”) plus する (“to do”). Grammatically, 旅行 becomes the object of する, so you add を. In polite speech you’ll often hear 旅行をします. Saying 旅行します isn’t strictly wrong—it’s a contracted form—but the standard breakdown uses を.

Why use 旅行をします instead of 旅行に行きます (“go on a trip”)?

Both are natural. 旅行をします literally means “do a trip,” emphasizing the travel activity itself. 旅行に行きます (“go to a trip”) focuses more on the act of going somewhere. The choice depends on nuance: if you want to stress “we’ll travel,” use 旅行をします; to stress “we’ll go,” use 旅行に行きます.

How do we know します refers to a future action and not the present?

Japanese doesn’t have a separate future tense. The present form します can mean both present and near future. Context—here 週末に (“on the weekend”)—tells us it’s referring to something they will do, not something they are doing right now.

Could we use 週末は instead of 週末に?

You can say 両親は週末は旅行をします, but that marks 週末 as a topic or point of contrast (“As for weekends…”), suggesting weekends vs. weekdays. If you simply want to specify “when” they travel, you use 週末に.

What’s the difference between は and が if I said 両親が週末に旅行をします?

が marks the subject that performs an action without introducing it as the topic of conversation. は makes 両親 the topic, framing the whole sentence around them. Both sentences are grammatical, but は is used when you’re giving information about “my parents” as the main point.