William Powell: So I'm a hero ... I was shot twice in the Tribune.
Myrna Loy: I read where you were shot five times in the Tabloids.
Powell: It's not true ... he didn't come anywhere near my Tabloids."
—From "The Thin Man"

Many jokes, such as the banter between William Powell and Myrna Loy above, depend on an ambiguity in the sentence structure. Loy means that she read the story in the Tabloids, but Powell plays on the idea that he was shot in the Tabloids, and therefore that the tabloids are a body part. In effect, Powell reanalyzes the original statement in order to make his joke. Such ambiguities are frequent in all sorts of language, not just jokes.

(15) The Red Cross evacuated the refugees from Sudan.

This sentence can be interpreted as saying either that the refugees were evacuated from Sudan or that they were from Sudan. You may be predisposed to read this sentence with the first interpretation. In fact, the second version may seem as if it means the same thing. But consider sentence (15) in the following context:

After Hurricane Katrina struck the New Orleans, Maria Veracruz, a long-time worker for the Red Cross, experienced a feeling of deja vu. When she arrived in the stricken city, she saw faces that she had encountered only six months before on the dusty plains of East Africa. With full appreciation of the irony, the Red Cross evacuated the refugees from Sudan for a second time.

With this larger context, we are now primed to read the sentence according to the second grouping. But whichever interpretation we apply to (15), it's important to notice that our interpretation is reflected in the constituency of the items in the sentence. We can show the structure of each interpretation visually by means of diagrams:

The first diagram shows a line extending from the prepositional phrase (PP) to the noun phrase (NP), indicating that the prepositional phrase is part of (i.e., a constituent of) the noun phrase. This grouping reflects the second interpretation above. Notice that not only does it imply a particular meaning—the refugees are originally from Sudan—but it also indicates that the complete string of words, the refugees from Sudan, acts as a unit. So, for example, if we ask, "Who did the Red Cross evacuate?" we would answer "The refugees from Sudan." Or if we expressed the idea in the passive voice, we would say

(15a) The refugees from Sudan were evacuated by the Red Cross.

The second diagram shows a line extending from the prepositional phrase directly to the verb phrase. This diagram reflects the first interpretation above: the refugees are evacuated from Sudan. By connecting the line directly to the verb phrase, we indicate that from Sudan gives information that modifies the verb evacuated rather than the noun refugees. Notice that in this interpretation, the noun phrase the refugees is also part of the verb phrase, but the noun phrase and the prepositional phrase do not form a single unit. For example, the passive form would be

(15b) The refugees were evacuated from Sudan by the Red Cross.

In other words, the string of words the refugees from Sudan does not behave as a single structural unit (constituent) under this reading of the sentence.

Sometimes, particularly once you become more familiar with syntax, the constituency of words in a sentence will be intuitively obvious. At other times, however, you will need to think carefully. To tell if words are constituents, if they are working together or not, you can try several tests.

Substitution is a particularly good test. If you can replace the candidate phrase with a pronoun (e.g., they or it) it's a noun phrase:

(16a) The golfers were forced off the course by the approaching lightning.
(16b) They were forced off the course by the approaching lightning.

Verb phrases can usually be replaced with do so:

(17) Yolanda has saved for retirement since her 20s. John has done so only since he married.

Prepositional phrases can often be replaced by a single word (traditionally identified as an adverb):

(18a) She went to the bar.
(18b) She went there.

You can also demonstrate phrase structure if the words will move as a unit. In other words, it is often possible to recast a sentence so that it still has more or less the same meaning but so that its elements appear in a different order. Sometimes, you can do this by simple rearrangement:

(19a) They found their guest waiting in the den.
(19b) In the den, they found their guest waiting.

Movement can also be shown by creating a so-called "wh-cleft" sentence.[1] Wh-cleft sentences are formed by rearranging a basic sentence in this fashion:

(20a) That woman left her abusive husband.
(20b) Her abusive husband is whom that woman left.

The cleft sentence has the form:

moved item + form of TO BE + wh-word + clause

Notice that only phrases move—you can't cleft a single word, or any other string of words that doesn't constitute a phrase:

(20c) *Husband is whom that woman left her abusive.
(20d) *Abusive husband is whom that woman left her.
(20e) *Her abusive is whom that woman left husband.

One other test that often works is to see if the candidate phrase could be the answer to a (normal) question.

(21) Where did they find their guest? In the den.

But there is no natural question about the content of the sentence that could elicit "found their" as an answer.[2]

These tests for constituency are important to understand when you come to analyze sentences for yourself, so it's a good idea to take some time to make sure you fully understand how to apply them.

Notes

[1] The name comes from the presence of a wh- word (who, why, etc.). There are also other types of cleft sentences.

[2] Of course you can ask questions such as "what are the second and third words of the sentence," but those aren't sentences about the content of the sentence.