Learn Hardcore Japanese: asita kawa de booto ni norimasu. - I will ride a boat on the river tomorrow.

In this sentence, 明日 is pronounced ashita, which is the most common reading in everyday conversation. The alternative reading asu is more formal or literary and you’ll see it in written announcements or headlines.

Why is there no particle after 明日?

Time expressions like 明日 (tomorrow) often stand alone at the start of a sentence without any particle. You can add に for extra emphasis or formality (e.g. 明日に), but in normal speech it’s dropped.

What does 川で mean, and why do we use the particle で here?

川で means “at (or on) the river.” The particle で marks the location where an action happens. So 川で乗ります tells us that the riding takes place at the river.

Why is ボート written in katakana instead of kanji or hiragana?

ボート is a loanword (外来語) from English “boat.” Japanese convention is to write foreign words in katakana to distinguish them from native Japanese vocabulary.

Why is に used in ボートに乗ります instead of を?

The verb 乗る (to ride/get on) takes に to mark what you are mounting or boarding. In contrast, を is used for direct objects of transitive verbs (e.g. 本を読む “read a book”).

What’s the plain form of 乗ります, and how would you say the sentence casually?

The polite form 乗ります corresponds to the plain (dictionary) form 乗る. In casual speech you’d drop the ます ending and say:
明日川でボートに乗る。