"Their grandchild is your cousin."

Translation: Cucu mereka adalah sepupu kamu.

[anno35]

how do you know whether or not to include 'adalah'? Is there any rule?

[drnuramita]

There's no really a rule. But, mostly when you describe something using another thing or noun (not adjective), it's best to include 'adalah'. In this sentence, we are describing 'their grandchild'.

Another example in using 'adalah' in describing thing :

This person is a teacher.
Orang ini adalah guru.

But, if you describe the thing with adjective, you don't need 'adalah'. Example :

This person is smart.
Orang ini pandai.

BUT, as I said before, there is no rule. In many occasions (mostly when speaking informally) you can omit 'adalah' when describing thing and Indonesian still understand the meaning. So you can say 'orang ini guru' in informally speaking.

[sumsel1]

Is 'adalah' required or would it be understood when omitting 'adalah'?

[J8SZ81]

Why do I have to use adalah and cannot use ada?

[EJNRr1]

I figure you would use "adalah" as an emphasis when using "is".

Darth Vader adalah ayah kamu!

[liduotyler]

In conclusion, if to describe a person/a thing, use adalah
if to describe by using adjective, there's no need to use adalah