haisya ha te wo syoudokusite musiba wo kezuri, watasi ha ugaisimasu.

You’re right: normal Japanese writing usually does not use spaces.

This sentence would normally be written like this:

歯医者は手を消毒して虫歯を削り、私はうがいします。

Spaces are often added in teaching materials to help beginners see the word boundaries more clearly.

Here, は marks the topic of each part of the sentence.

• 歯医者は = as for the dentist
• 私は = as for me

Using は twice creates a contrast:

• the dentist does one set of actions
• I do another action

So the sentence has a structure like:

• As for the dentist, he disinfects his hands and drills the cavity,
• and as for me, I rinse.

This use of は is very common when switching focus from one person to another.

が would focus more on who is doing the action.
は sets up the dentist as the topic and gives background information about what happens.

In this sentence, the point is not who is the one doing it? but rather what happens with the dentist, and what happens with me. That makes は more natural.

Also, because the sentence contrasts 歯医者は with 私は, は works especially well.

In Japanese, body parts are often mentioned without a possessive when the owner is obvious from context.

So:

• 手を消毒して naturally means disinfecting his hands here

Because the subject is the dentist, Japanese does not need to say his hands explicitly.

This is very common:

• 手を洗う = wash one’s hands
• 目を閉じる = close one’s eyes
• 顔を上げる = raise one’s face / lift one’s head

English usually needs the possessive, but Japanese often does not.

消毒して is the て-form of 消毒する.

The て-form connects actions, and here it means something like:

• disinfects his hands and then...
• after disinfecting his hands...

So:

手を消毒して虫歯を削り
= disinfect his hands and drill the cavity

The て-form often links actions in sequence.

This is a great question. 削り is the continuative/conjunctive form of 削る. It connects to the next clause, similar to 削って.

So these are close in meaning:

• 虫歯を削って、私はうがいします
• 虫歯を削り、私はうがいします

The version with 削り、 tends to sound a bit more written, formal, or narrative.
Learners often see て-form connections first, but Japanese also uses this verb-stem + punctuation style, especially in writing.

Literally, 虫歯を削る means to shave/drill away the cavity / decayed part.

In real life, yes, the dentist is working on the tooth, but Japanese often expresses it as removing the decay rather than drilling the tooth.

So 虫歯を削る is a normal Japanese way to describe dental treatment.

うがいします means to gargle or to rinse the mouth, depending on context.

At the dentist, it usually means rinsing your mouth.

Grammatically, うがい is a noun, and する turns it into a verb:

• うがいする
• polite: うがいします

You may also see:

• うがいをする
• うがいをします

Both are correct. In everyday Japanese, うがいする is very common.

In Japanese, when several actions are connected in one sentence, only the final verb usually carries the full tense and politeness marking.

Here:

• 消毒して = connecting form
• 削り = connecting form
• うがいします = final polite verb

So the sentence builds up a chain of actions, and the last verb finishes the sentence.

This is very normal Japanese sentence structure.

Japanese non-past forms like します can mean:

• present/habitual
• near future
• general procedure

In this sentence, it most naturally sounds like a description of what happens during dental treatment, or a habitual/procedural sequence:

• the dentist disinfects his hands
• drills the cavity
• and I rinse

So it is not necessarily talking about right this second. It can describe a routine or expected sequence.

歯医者 can mean either:

• the dentist
• the dentist’s office / dental clinic

But in this sentence, it clearly means the dentist as a person, because 歯医者 is the one doing the actions:

• disinfecting hands
• drilling the cavity

If it meant the clinic, that would not make sense here.

The comma helps show a pause and makes the sentence easier to read.

It also marks a shift from:

• what the dentist is doing

to

• what I am doing

So the comma supports the contrast between 歯医者は and 私は.

In Japanese, punctuation is somewhat flexible, but this comma is very natural here.