paathii de kanpaisita.

パーティー is a loanword borrowed from English party, so it is written in katakana, the script commonly used for foreign words.

A few pronunciation notes:

• パー = paa
• ティー = tii

So パーティー sounds like paatii, not exactly like English party.

Here, で marks the place or setting where the action happens.

So in パーティーで乾杯した, で means something like:

• at the party
• during the party
• in the party setting

It tells you where the toast happened.

Japanese often uses で for the place where an action is carried out.

Compare:

• 学校で勉強した。 = studied at school
• レストランで食べた。 = ate at a restaurant
• パーティーで乾杯した。 = made a toast at a party

に is often used for:

• destination: 学校に行く = go to school
• time: 3時に行く = go at 3 o'clock
• existence target/recipient and some other functions

Since 乾杯した is an action happening in a location, で is the natural choice.

It is basically a noun meaning toast or cheers, but it can also work as a suru-verb.

So:

• 乾杯 = a toast / cheers
• 乾杯する = to make a toast / to say cheers
• 乾杯した = made a toast / said cheers

This is very common in Japanese: a noun + する becomes a verb.

乾杯した comes from:

• 乾杯する = to toast / to say cheers
• past tense of する = した

So:

• 乾杯する → non-past
• 乾杯した → past

That means the sentence is in the plain past tense.

Japanese often leaves out the subject when it is clear from context.

So パーティーで乾杯した。 could mean:

• I toasted at the party.
• We made a toast at the party.
• They said cheers at the party.

The exact subject depends on the situation. This is completely normal in Japanese.

Yes. In Japanese, this is a perfectly normal complete sentence.

Japanese frequently omits:

• the subject
• the object
• other information that is understood from context

As long as the listener can infer the missing information, the sentence sounds natural.

With many suru-verbs, the を can be omitted.

So both of these are possible:

• 乾杯した
• 乾杯をした

The version without を is very common and natural.

In everyday Japanese, 乾杯した is simpler and more typical.

Yes. 乾杯！ is a very common expression meaning Cheers!

So there are two related uses:

• 乾杯！ = Cheers!
• 乾杯する / 乾杯した = to make a toast / made a toast

In your sentence, it is the verb use: made a toast.

Not necessarily. 乾杯する means to make a toast or say cheers, not specifically to drink alcohol.

Usually people do raise a glass and drink, but the core meaning is the ceremonial toast. The drink could be alcoholic or non-alcoholic depending on the situation.

Yes. Normal Japanese writing would usually be:

パーティーで乾杯した。

The spaces in learning materials are often added just to make the sentence easier to read for beginners.

It is natural and grammatical, but a little short because Japanese often drops context.

Depending on the situation, a speaker might also say:

• パーティーでみんなで乾杯した。 = We all made a toast at the party.
• パーティーの最初に乾杯した。 = We made a toast at the beginning of the party.

But パーティーで乾杯した。 by itself is perfectly fine.

Yes. You can say what the toast is for with に.

For example:

• 新郎新婦に乾杯した。 = We toasted the bride and groom.
• 成功に乾杯した。 = We drank to success.

In your sentence, that target is simply not mentioned. It just says that the toast happened at the party.