She’s just been promoted, hence the new car.

Hence is an adverb which has the meaning ‘and that is why’; ‘that’s an explanation for’; ‘as a consequence’ or ‘for this reason’.

There are several possibilities but it is often placed at the beginning of the second clause in a sentence, and is followed by a noun or a verb.

Because it can be used in place of longer expressions, as above, it makes sentences shorter and more succinct, and so it is useful for improving writing style.

• We wanted better communication with our customers, hence the decision to be more present on social media.

• With more people working from home, there are fewer cars on the road and, hence, less pollution.

• Sleeping patterns depend on age, hence the reason why teenagers usually go to bed later and get up later.

• Brainstorming stimulates some unexpected ideas and hence facilitates innovation.

• They’re working on some cutting cutting-edge technology in their laboratories, hence all the extra security.

• I didn’t have time to proofread, hence the typos!

Note that the word ‘hence’ has other meanings that are different from this one. If in doubt, check in a good dictionary.