Remember that multi-word prepositions include:
- Ahead of
- In front of
- Instead of
- Near to
- In time for
- As well as
...and many more!
You might notice that a few of those groups contain abstract nouns (in front of, in time for) or adverbs (ahead of, as well as). (And there's that slippery word as again!)
Why not call them preposition groups? Because they're not flexible in the same way that noun, verb and adjective groups are—they're basically fixed units of speech.
Then why not call them phrases? We could, but then we'd get mixed up with the larger prepositional phrases we've been talking about throughout this lesson. So it's easier just to call them prepositions.