this + [singular noun] (near) / that + [singular noun] (far)
these + [plural noun] (near) / those + [plural noun] (far)
Use these words to show which thing you mean. The choice depends on two things: is it near or far, and is it one or more than one?
The word can stand alone, without a noun. One and ones take the place of the noun too.
When you introduce someone, say This is ….
On the phone, you can say who you are with this is or it's: Hello, this is Yuki.
That can point to what someone has just said or done.
This is for the time you're in now or the one coming: this week, this morning, this summer. That is for a time in the past or far away: that day.
This has a short vowel and ends in "s". These has a long vowel and ends in "z", like "theez".