Вставь is, are.

This a computer.

Those desks.

These lamps.

That a chair.

It Dan's rocking horse.

They Lida's toys.

This is a computer.

Those are desks.

These are lamps.

That is a chair.

It is Dan's rocking horse.

They are Lida's toys.

Местоимения this, that и it указывают на единственное число, то есть глагол is.

Местоимения these, those и they указывают на множественное число, то есть глагол are.

This is

Those are

These are

That is

It is

They are