In the afternoon light is a phrase that does not tell a reader much. The reader is looking around for more information, and some active element. Questions arise, such as what is in the afternoon light? Who is in the afternoon light (if anyone)? What is happening in the afternoon light? A sentence, on the other hand, can stand on its own.
Her face is reflected. This is a sentence because there is a subject and verb although no defined object as yet. What is reflecting her face? A mirror?
If you add the former phrase to the sentence, the result is: Her face is reflected in the afternoon light. Now you know what is illuminated in the afternoon light (or rather reflected as a poetic inflection). You know it is a her and the assumption would be that she is a woman, but she could just as easily be an animal other than human. And you also know what is reflecting her face. The whole construction is now a sentence using a phrase as the object. You can throw in some adjectives to define things more, for example, Her smooth white face is reflected in the crimson afternoon light. Add a second phrase and you have a complete picture: Her smooth white face is reflected in the crimson afternoon light of the setting sun.
Phrases are short snippets that lack a verb, but make sense when paired with a sentence. A trap for the unwary is the clause that masquerades as a sentence, but in reality must also be paired to a sentence.
When she stands on the hill. This is an example of an adverbial clause of time. Clauses can be adjectival or adverbial and commonly begin with when, who, where, that and which. Whoa! Now things are getting complicated. Don’t worry too much about the terms; I get mind boggled as well sometimes. Returning to this example, because the structure begins with when, there must be some other action associated with it. I can actually attach this clause to the sentence I’m building. We now have: Her smooth white face is reflected in the crimson afternoon light of the setting sun, when she stands on the hill.
You could also attach the clause to the beginning of the sentence although it may offend grammar purists. I’m something of a grammar nazi myself, but have come to appreciate that in some contexts, writers can bend and flex the rules a little. The result then would be: When she stands on the hill, her smooth white face is reflected in the afternoon light of the setting sun.
I may risk adding just one more clause to this sentence – this time an adjectival clause qualifying an aspect about the sun. In this case I must add the clause directly after the word sun at the end of the latter sentence. Here is the clause: That hovers poised over the sea. I have introduced another piece of information – the fact that the location of the hill is near the sea. You also know that this is a western facing coast as the sun is setting over the sea.
The completed sentence then becomes: When she stands on the hill, her smooth white face is reflected in the afternoon light of the setting sun that hovers poised over the sea.
Phrases and clauses are an important part of sentences. They impart extra information that helps to paint a full picture for the reader.