"This is my girlfriend; she likes to sleep."
Translation:Это моя девушка, она любит спать.
But the word это here isn't a determiner, or a word which refers to which girlfriend you are talking about (this one or that one), but a statement-maker, which is a word which introduces who or what are you talking about (it/this/that/these/those is/are a/an/the ...).
Это as statement maker never agrees with the following word;
- Это шапка (This is a hat)
- Это насок (This is a sock)
- Это пальто (This is a coat)
- Это брюки (These are some trousers)
But это as a determiner always does;
- Эта шапка (This hat)
- Этот насок (This sock)
- Это пальто (This coat)
- Эти брюки (These trousers)