Hi,

the only things I'd add to that are:

generally, these expressions concern measurement. Typically, they're made up of a number and then a category of measurement or component of something:

a four-hour walk
a one-armed man

etc

The convention in writing has been to hyphenate them - probably to show that they function as a single descriptive word.

There are plenty of fun activities that teachers can do with this area of grammar / vocab. Here are a couple:

It's pretty easy to make up dominoes for this: eg a domino with 'man' in the left square and 'two-kilogram' in the right. The student has to find the domino to put on the right - one with 'weight' in the left square and 'three-minute' on the right etc.

Pictionary also works well. Divide the class into teams. One person from each team stands up. You give them one of the expressions eg 'a one-armed man' and then they have to rush to the board and draw a picture. The first team to shout out the right expression for their picture gets a point.

You could also get students to do a poster competition. You give them (or they choose) five different expressions (eg a ten-foot wall, a five-kilogram fish, a two-headed dog etc) and get students in groups to produce a poster representing all of them. Similarly, instead of a poster, they have to come up with a story ...

etc etc!!