Karang gigi itu keras.

No. Karang gigi is a fixed expression meaning tartar or dental calculus.

Even though:

• karang can mean coral or reef
• gigi means tooth/teeth

when they are put together as karang gigi, the meaning is idiomatic: hardened deposits on the teeth.

So learners should treat karang gigi as one vocabulary item.

Indonesian often does not use a verb like to be in simple present sentences.

So:

• Karang gigi itu keras = That tartar is hard / Tartar is hard

Literally, Indonesian is just:

• karang gigi itu = that/the tartar
• keras = hard

This is completely normal in Indonesian. You do not need an equivalent of is here.

Itu most basically means that, but after a noun it can also make the noun sound more definite, sometimes close to the in English.

So karang gigi itu can mean:

• that tartar
• the tartar
• sometimes just tartar in a specific context

In many everyday sentences, itu after the noun points to something already known in the conversation.

In Indonesian, demonstratives like ini and itu usually come after the noun.

So:

• buku itu = that book
• rumah ini = this house
• karang gigi itu = that tartar

This is the normal Indonesian word order.

Not only physically hard, but in this sentence that is the meaning.

Keras can mean:

• hard physically
• tough
• strict
• harsh
• loud in some contexts

But in Karang gigi itu keras, it clearly means physically hard.

Gigi can mean tooth or teeth, depending on context.

Indonesian nouns usually do not change form for singular vs. plural the way English nouns do.

So:

• gigi = tooth or teeth
• karang gigi refers to tartar on the teeth in general

You understand the number from context, not from a special ending.

Yes, it can be possible, but the nuance changes.

• Karang gigi itu keras = that/the tartar is hard
• Karang gigi keras = tartar is hard or dental tartar is hard, more general

Adding itu makes it sound more specific or more tied to something already being referred to.

No. Indonesian adjectives do not agree with the noun in gender, number, or anything like that.

So keras stays keras no matter what noun it describes or predicates about.

Examples:

• batu itu keras = that rock is hard
• roti itu keras = that bread is hard
• karang gigi itu keras = that tartar is hard

The adjective does not change.

Here keras is the predicate: it tells you a quality of karang gigi itu.

So the structure is:

• Karang gigi itu = subject
• keras = predicate adjective

It is not a phrase like hard tartar.
If you wanted hard tartar as a noun phrase, Indonesian often still allows similar wording, but in this sentence the intended reading is clearly The tartar is hard.

You would usually use tidak before the adjective:

• Karang gigi itu tidak keras. = That tartar is not hard.

This is because keras is an adjective, and tidak is the normal negator for adjectives and verbs.

A simple pronunciation guide is:

• karang ≈ kah-rang
• gigi ≈ gee-gee
• itu ≈ ee-too
• keras ≈ kuh-rass

A few helpful notes:

• ng in karang is like the ng in sing
• g in gigi is always a hard g, like in go
• Indonesian spelling is quite regular, so words are usually pronounced close to how they are written

No. That would not be the natural reading.

Because karang gigi is a set expression meaning tartar/dental calculus, the sentence is understood as talking about that substance, not about teeth themselves.

If you wanted to say The teeth are hard, you would say:

• Gigi itu keras

But in your sentence, karang gigi should be read together as one term.