Kalimat-kalimat ini mudah dimengerti.

This is reduplication, a very common way to show plurality in Indonesian.

• kalimat = sentence
• kalimat-kalimat = sentences

So kalimat-kalimat ini means these sentences.

Indonesian does not always have to mark plurals, especially if the meaning is already clear from context, but reduplication is a standard and natural way to make plural meaning explicit.

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

• kalimat ini = this sentence
• kalimat-kalimat ini = these sentences

This is the normal Indonesian word order. So even though English says these sentences, Indonesian says the equivalent of sentences these.

Indonesian usually does not use a verb like to be in simple present-tense sentences.

So:

• Kalimat-kalimat ini mudah dimengerti
  literally works like
• These sentences easy understood

That is perfectly normal in Indonesian. You do not need adalah here.

In fact, using adalah in this sentence would sound unnatural. Adalah is used in more limited situations, usually in formal definitions or emphasis, not in everyday adjective predicates like this one.

mudah means easy, and dimengerti means understood or to be understood.

So mudah dimengerti is literally something like:

• easy to be understood
• more natural English: easy to understand

This pattern is very common in Indonesian:

• mudah dipelajari = easy to learn
• sulit dipercaya = hard to believe
• gampang dilihat = easy to see

So mudah + di-verb often corresponds to English easy to + verb.

Because mengerti is the active form, while dimengerti is the passive form.

• Saya mengerti kalimat ini = I understand this sentence
• Kalimat ini dimengerti = This sentence is understood

In mudah dimengerti, the sentence itself is the thing being understood, so the passive form is the natural choice.

English also does something similar in meaning:

• These sentences are easy to understand

Even though English uses to understand, the sense is still that someone understands the sentences, not that the sentences understand something.

No. They are different things.

• di as a preposition is written separately:

  • di rumah = at home
  • di sekolah = at school

• di- as a prefix for passive verbs is written attached to the verb:

  • dimengerti
  • dibaca
  • ditulis

So dimengerti is one word because di- here is a verb prefix, not a preposition.

Yes, absolutely. That is also natural.

Both are common:

• mudah dimengerti
• mudah dipahami

A rough difference is:

• mengerti / dimengerti often feels a bit more like understand
• memahami / dipahami can feel slightly more like comprehend / grasp fully

But in many everyday contexts, they overlap a lot, and both can be translated as understand.

So this sentence with dipahami would still be very normal.

No. Indonesian adjectives do not change for number.

So:

• Kalimat ini mudah dimengerti = This sentence is easy to understand
• Kalimat-kalimat ini mudah dimengerti = These sentences are easy to understand

The adjective mudah stays the same whether the noun is singular or plural.

Yes, but then the meaning would normally be This sentence is easy to understand, not These sentences are easy to understand.

If you want the plural meaning clearly, kalimat-kalimat ini is the straightforward form.

That said, Indonesian sometimes leaves nouns unmarked for plural if context already makes it clear. But in isolation, if you say kalimat ini, most learners should understand it as singular: this sentence.