senmendai no mae de hamigakiko wo dasisugita.

A natural reading is:

せんめんだい の まえ で はみがきこ を だしすぎた。

• 洗面台 = せんめんだい
• 前 = まえ
• 歯みがき粉 = はみがきこ
• 出しすぎた = だしすぎた

洗面台の前で means in front of the sink / washstand.

It breaks down like this:

• 洗面台 = sink, washstand, bathroom sink area
• の = links nouns, here meaning of or forming in front of
• 前 = front
• で = marks the place where an action happens

So:

• 洗面台の前 = the area in front of the sink
• 洗面台の前で = at/in front of the sink, where the action took place

Because で marks the place where an action happens.

In this sentence, 出しすぎた is an action, so で is the natural choice:

• 洗面台の前で 歯みがき粉を出しすぎた。 = I squeezed out too much toothpaste in front of the sink.

By contrast, に is more often used for:

• existence: そこにある = it is there
• destination: 学校に行く = go to school
• specific point in time: 3時に = at 3 o’clock

So で is correct because this is the location of an action.

歯みがき粉 means toothpaste.

It is made of:

• 歯 = tooth/teeth
• みがき = brushing/polishing
• 粉 = powder

Historically, the word literally meant something like tooth-brushing powder, which is why 粉 is in the word even though modern toothpaste is not powder.

As for the writing:

• 歯磨き粉 is a more fully kanji version
• 歯みがき粉 is a very common mixed writing
• はみがきこ is also possible in hiragana

Japanese often uses mixed kanji and kana for readability or style.

出しすぎた is made from:

• 出す = to take out, put out, squeeze out
• すぎる = to do too much, to exceed
• た = past tense

So:

• 出す → 出し (stem)
• 出し + すぎる = 出しすぎる = to squeeze out too much / to put out too much
• 出しすぎた = squeezed out too much

In this sentence, it means the speaker used too much toothpaste.

Because すぎる attaches to the verb stem.

For an -u verb like 出す, the stem is formed by changing す to し:

• 出す → 出し
• 出しすぎる

This pattern is very common:

• 食べる → 食べすぎる = eat too much
• 飲む → 飲みすぎる = drink too much
• 言う → 言いすぎる = say too much

So 出しすぎた follows a normal grammar pattern.

を marks the direct object of the verb.

Here, the thing being squeezed out is 歯みがき粉:

• 歯みがき粉を出す = to squeeze out toothpaste

So in the full sentence:

• 歯みがき粉を出しすぎた = squeezed out too much toothpaste

Yes. In Japanese, 出す is very natural for things like squeezing or dispensing something out of a container.

For example:

• 歯みがき粉を出す = squeeze out toothpaste
• シャンプーを出す = dispense shampoo
• ティッシュを出す = take out a tissue

In English, we might say squeeze out, but Japanese often just uses 出す.

Yes, the subject is omitted.

Japanese often leaves out the subject when it is obvious from context. In a sentence like this, the default interpretation is usually:

• I squeezed out too much toothpaste

But depending on context, it could also mean:

• you squeezed out too much toothpaste
• he/she squeezed out too much toothpaste

If this sentence appears by itself, most learners should understand it as I squeezed out too much toothpaste, because that is the most natural guess in everyday speech.

Yes. 〜すぎた adds the idea of excess or overdoing it.

Compare:

• 歯みがき粉を出した。 = I squeezed out toothpaste.

• 歯みがき粉を出しすぎた。 = I squeezed out too much toothpaste.

So 〜すぎた often implies:

• more than intended
• more than necessary
• a small mistake or mild regret

In this sentence, it sounds like the speaker realized they used too much.

Yes.

You could simply say:

• 歯みがき粉を出しすぎた。 = I squeezed out too much toothpaste.

Adding 洗面台の前で gives scene-setting information. It tells you where it happened, but it is not required for the sentence to be grammatical.

Often yes, but not perfectly.

洗面台 usually refers to the bathroom sink area or washstand, not just the basin itself. It can include the sink, counter, mirror area, and fixture as a unit.

So depending on context, translations like these can all work:

• sink
• bathroom sink
• washstand
• vanity

In this sentence, in front of the sink is a very natural translation.

No, here it is clearly location.

前 can sometimes mean before in time, but that use usually appears differently, such as:

• 寝る前に = before sleeping

In this sentence:

• 洗面台の前で

Since 洗面台 is a physical place and で marks the location of an action, 前 clearly means in front of.

It is casual/plain style.

The ending 出しすぎた is plain past.

A more polite version would be:

• 洗面台の前で歯みがき粉を出しすぎました。

That would be more suitable in polite conversation.

The structure is:

[place] で [object] を [verb]

In this sentence:

• 洗面台の前で = place where the action happened
• 歯みがき粉を = object
• 出しすぎた = verb

So the pattern is very standard Japanese word order:

• location
• object
• action

Even though English word order is different, this is a common and useful pattern to recognize in Japanese.