"मेरे दादा छत पर हैं।"

Translation: My grandfather is on the terrace.

[CathyIsAwesome]

I said patio instead of terrace and d it was marked wrong. Aren't they the same thing?

[RanzoG]

lol, no.
छत is ROOF, and also terrace due to the structure of lots of Indian houses, yes. But if you remember it as roof, you won't confuse it with patio :)

[sunitakuma9]

Before (on) is not use of (are)???

[OJFord]

Do you mean after on, पर, why is it हैं with the ं?

Because it's respectful to दादा जी to use the plural 'are' instead of singular 'is'.

[SpanishNakshatra]

Why "my grandfather is on terrace" wrong ? It should be correct.

[OJFord]

In English you can't just say 'on terrace', what terrace? It has to be a/the/that/this/your/my/something terrace. In this case, 'the', because Hindi doesn't have an equivalent (called a 'definite article' in grammar) whereas the other possibilities would have एक/यह/etc.