tearai ha kansen wo yobousimasu.

It is read:

てあらい は かんせん を よぼうします

A natural pronunciation guide would be:

tearai wa kansen o yobō shimasu

A few notes:

• 手洗い = てあらい
• は is written ha but here it is pronounced wa because it is the topic particle
• を is often pronounced o
• 予防 = よぼう, with a long おう sound at the end

The spaces are just for learners. In normal Japanese, this would usually be written as:

手洗いは感染を予防します。

Japanese normally does not use spaces between words the way English does. Textbooks and learning materials often add spaces to make grammar easier to see.

手洗い means handwashing or washing one’s hands as a general activity.

In this sentence, it works like a noun, so handwashing is the best fit.

Compare:

• 手洗いは大切です。 = Handwashing is important.
• 手を洗います。 = I wash my hands / wash your hands / wash hands

So:

• 手洗い = the act/practice of handwashing as a concept
• 手を洗う = to wash hands

は marks the topic of the sentence.

So 手洗いは means something like:

• As for handwashing, ...
• Speaking of handwashing, ...

The sentence is telling you something about the topic handwashing: namely, that it prevents infection.

This is very common in Japanese. English often starts directly with the subject, but Japanese often introduces a topic first.

を marks the direct object of the verb.

The verb here is 予防します = prevent.
What is being prevented? 感染 = infection.

So:

• 感染を予防します = prevent infection

You can think of を as marking the thing that receives the action of the verb.

予防します means prevents or will prevent, depending on context.

It comes from:

• 予防 = prevention
• する = to do

Together:

• 予防する = to prevent
• 予防します = polite form of 予防する

This pattern is very common in Japanese:

• 勉強する = to study
• 運動する = to exercise
• 予防する = to prevent

So 予防します is simply the polite present/future form.

Technically, 予防します can cover both present and future, depending on context.

Here, it is best understood as a general truth:

• Handwashing prevents infection.

Japanese non-past forms often express:

• habitual actions
• general facts
• future actions

So this sentence is not really talking about one specific future event. It is stating a general fact.

Yes, 手洗いが感染を予防します is possible, but it changes the nuance.

• 手洗いは感染を予防します
  = As for handwashing, it prevents infection.
  This sounds like a general statement about handwashing.

• 手洗いが感染を予防します
  = Handwashing is what prevents infection.
  This can sound more focused or contrastive, as if answering What prevents infection?

For a general statement in educational or health-related language, は is very natural.

感染 means infection.
感染症 means infectious disease.

They are related, but not exactly the same.

• 感染 focuses on the act/state of becoming infected
• 感染症 refers more specifically to an infectious illness or disease

So 感染を予防します means prevents infection, which fits well in a health-safety statement.

Because は usually attaches to a noun or noun-like phrase, not directly to a plain verb phrase like 手を洗う in this kind of sentence.

Japanese often turns an action into a noun when making general statements.

So instead of:

• 手を洗うは ❌

Japanese uses a noun:

• 手洗いは ✅

If you want to use the verb phrase, you would need a different structure, such as:

• 手を洗うことは大切です。 = Washing your hands is important.

Here, こと turns the verb phrase into a noun-like expression.

Yes. It ends with 予防します, which is the polite -ます form.

Compare:

• 予防します = polite
• 予防する = plain / dictionary form

So:

• 手洗いは感染を予防します。 = polite, appropriate for public information, signs, announcements, school materials, etc.
• 手洗いは感染を予防する。 = plain style, more like a textbook statement, note, or casual explanatory style

Here is a quick breakdown:

• 手 = hand

• 洗い = washing
  → 手洗い = handwashing

• 感染

  • 感 = feeling/sense, but in this word it is just part of the established compound
  • 染 = dye/stain/infect
    → 感染 = infection

• 予防

  • 予 = beforehand/in advance
  • 防 = defend/prevent
    → 予防 = prevention

This kind of kanji breakdown can help memory, but it is most important to learn the words as complete vocabulary items.

A helpful way to map it is:

• 手洗いは = As for handwashing,
• 感染を = infection
• 予防します = prevents

So the overall structure is:

As for handwashing, it prevents infection.

In natural English, we would usually just say:

Handwashing prevents infection.

That is a good example of how Japanese topic structure and English sentence structure do not always match word-for-word.