They argue that English has two main aspects “which combine fairly freely with tense and mood: the ’perfect’ (e.g. I have/had read the book, I will/would have read the book) and the ‘progressive’ (I am/was reading the book, I will/would be reading the book). They also combine freely with one another (I have/had been reading the book)”.
The scholar s also speak of other aspectual distinctions, e.g. the ‘habitual’ (I used to read), the ‘mutative’ (e.g. I got killed).
The perfect form builds up its own category, different from both the "tense" (present — past — future) and the "aspect" (continuous — indefinite), and not reducible to either of them.
The functional content of the category of "time correlation" (« временная отнесенность ») was defined as priority expressed by the perfect forms in the present, past or future contrasted against the non-expression of priority by the non-perfect forms.
2) the speaker’s reluctance or inability to use the Agent. The speaker may not want to mention the Agent for the sake of tact, or he/she may want to avoid making explicit reference to the Agent and thus give the writing a more objective flavour, or he/she simply may not know who carried out the process.