Shouldn't it be "Elle a dû l'oubliée?" [She must have forgotten it.]
Here, Oublier is infinitive form of the verb which means - to forget. So, in this example isn't it translated to - She must have forget about it, which doesn't sound right. How can it be translated to '...forgotten it.' which is past tense?
Aashpreet,
the infinitive form of ‘oublier’ is correct here.
Remember, that (the English) translation is just a representation of the meaning of the original language (French) phrase, and a direct word for word translation does not inherently have to be correct or make sense in the ‘recipient’ language.
The infinitive form of the verb is used in different situations in French. See links below for detailed discussion :
There are 2 elements that call for the infinitive use here -
1. It follows a conjugated verb form
and
2. In this case, that conjugated verb is also a semi-auxiliary form - the verb ‘devoir’ (most often used as semi-auxiliary; it changes meaning when it is not a semi-auxiliary form, being followed by a noun instead - see final link)
The tense is set by the first verb in the sequence ‘a dû’ - the imperative verb itself is an impersonal verb form.
https://progress.lawlessfrench.com/revision/glossary/verbal/infinitif-infinitive
https://www.lawlessfrench.com/grammar/infinitive/
https://www.fluentu.com/blog/french/french-infinitive/
https://www.lawlessfrench.com/grammar/verb-mood/
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