shikaeiseishi ga setsumeishimasu.

The dental hygienist will explain.

Breakdown of shikaeiseishi ga setsumeishimasu.

• が: subject particle
• 説明せつめいする: to explain
• 歯科衛生士しかえいせいし: a dental hygienist

Questions & Answers about shikaeiseishi ga setsumeishimasu.

What does が do in this sentence?

が marks the grammatical subject of the sentence—the person who is doing the action.

So in:

歯科衛生士が説明します。

the 歯科衛生士 is the one who explains.

A natural breakdown is:

• 歯科衛生士 = dental hygienist
• が = subject marker
• 説明します = explains / will explain

So the structure is basically:

[The dental hygienist] [is the one who] explains.

Why is が used instead of は?

This is a very common question.

Both が and は can appear with a noun before a verb, but they do different jobs:

• が marks the subject
• は marks the topic

With が, the sentence sounds more like it is identifying who does the explaining:

• 歯科衛生士が説明します。
  = The dental hygienist will explain. / It is the dental hygienist who will explain.

With は, the sentence would sound more like you are talking about the dental hygienist as a topic:

• 歯科衛生士は説明します。
  = As for the dental hygienist, they will explain.

In many situations both are possible, but が often feels more natural when introducing or specifying the person who does the action.