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One of my French workbooks has these two sentences in separate sections:

J'ai vu les moutons traverser la route.

La police l'a aperçu entrant dans un laboratoire médical.

It seems to me that the usage between traverser and entrant is the same here-- why is one infinitive and the other present participle?

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I don't know what your mother tongue is but, as you wrote your question in English, I guess you master this language. And in English the tenses used in a similar situation would be the same, i.e. "I saw some sheep cross the road" because I observed the process, and "The police spotted/saw him entering the doctor's surgery" because they saw him while he was performing that action. –  Paola Apr 22 '13 at 0:44

1 Answer 1

up vote 6 down vote accepted

In the first sentence, the important observation is that the sheep crossed the road. What was seen is the crossing. The fact that I've seen sheep is (mostly) irrelevant.

In the second sentence, the relevant is that the police spotted him. What was seen is him. The last part of the sentence merely specifies the conditions, but they are considered less relevant. It could be rephrased like this:

La police l'a aperçu. Il était en train d'entrer dans un laboratoire médical.

The following is also correct and slightly different:

La police l'a aperçu entrer dans un laboratoire médical.

The focus here is on “he entered the lab”. It lessens the importance of the fact he was spotted. It might even suggest that the trespass is the (or part of the) transgression.

In particular, the spelling somewhat reveals the difference in the plural case (for example):

La police les a aperçus entrant dans le labo.
La police les a aperçu entrer dans le labo.

In the first the object is “les”, in the second it's a full infinitive proposition.

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I couldn't have written a better explanation! –  Hugo Dozois Apr 19 '13 at 17:35
Yeah, this is a really excellent answer. Props. I was expecting to struggle with this one and you really elucidated it. –  Aerovistae Apr 26 '13 at 17:52

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