This, that, these, those

1. Function

The demonstratives this, that, these, those ,show where an object or person is in relation to the speaker.
This (singular) and these (plural) refer to an object or person near the speaker. That (singular) and those (plural) refer to an object or person further away. It can be a physical closeness or distance as in:

• Who owns that house? (distant)
• Is this John’s house? (near)

Or it can be a psychological distance as in:

• That‘s nothing to do with me.. (distant)
• This is a nice surprise! (near)

2. Position

• Before the noun.
• Before the word ‘one’.
• Before an adjective + noun.
• Alone when the noun is ‘understood’.