Breakdown of Nous observons les loups fatiguant le troupeau.
nous
we / us (object pronoun)
le
him, it (masculine direct object pronoun)
les
them (direct object pronoun)
le loup
the wolf
Questions & Answers about Nous observons les loups fatiguant le troupeau.
Why is fatiguant spelled with a "u" here, and why doesn't it have an "s" to agree with les loups?
In this sentence, fatiguant is a present participle (acting as a verb), not a verbal adjective. Present participles of verbs ending in -guer (like fatiguer) keep the "u". Also, present participles are invariable, meaning they never change their endings to agree with the noun they relate to.
How can I tell that fatiguant is acting as a verb (present participle) rather than an adjective here?
The biggest clue is that it has a direct object. The wolves are actively doing an action to something else: they are tiring the herd (le troupeau). If an -ant word is followed by a direct object, it is a present participle.
How would the sentence change if we just wanted to say "We observe the tiring wolves" without mentioning the herd?
You would write Nous observons les loups fatigants. Without a direct object, it becomes a verbal adjective describing the wolves. Notice that it drops the "u" (a special spelling rule for adjectives derived from -guer verbs) and adds an "s" to agree with the plural noun les loups.