The abstract is usually in the past tense due to it showing what has already been studied ... It can be beneficial to use passive when one wants to keep a similar subject in a series of sentences ...

The perfect tense is used to say what you did or what you have done in the past. To form the perfect tense you need a subject, an auxiliary verb and a past participle. Perfect tense with 'être ...

Use the past tense to report what happened in the past ... (Here, the agent — John — is the grammatical subject of the sentence.) In contrast, the passive voice focuses on the object that ...

Up until the late 80's the subject is generated as a specifier of T; we now consider such forms to be adjoined to TP: Note that there is no phonological (morphological) form assigned to to T:[-Past].