Б. С. Хаймович, Б. И. Роговская теоретическая грамматика английского языка

If you are writing home, give them my love.
It sometimes occurs with verbs which are but rarely used in (he continuous aspect ³ to stress the transitory, temporary nature of the action.
E. g. How are you l i k i n g your new job? (Hornby). Is it very tough going? he asked sympathetically. Are you hating it? (Randall).
When used with non-terminative verbs the present contin­uous non-perfect is really synonymous with the present non-continuous non-perfect.
Cf. I live in London.
I am living in London.
The difference lies in the fact that I live merely states an act, while I am living presents it in its progress and sounds more emotional and vivid.
_______________________
¹See p. 161
²See p. 134
³See p. § 221

The Past Continuous Non-Perfect

§ 269. As a language unit this grammeme denotes an action in the past (past tense), viewed in its development (continuous aspect) and unspecified as to its being prior to some situation or not (non-perfect order).

When used in speech the past continuous non-perfect may serve as a framework for another action.
E. g. The astounded male discovered that she was hav -ing a small rebellion of her own. (Lewis).
Like the present continuous non-perfect it may be linked with such adverbs as always, constantly, ever, continually, perpetually, etc., in which case it denotes a continuous action presented with a marked emphatic force.
E. g. He w a s always finding fault with everybody and they came to hate him very heartily. (Lovel).
It may occur with a relative meaning to denote a future action viewed from the past as in
They told me the Italians were leaving the next morning. (Hemingway).
The past continuous non-perfect and the past non-con­tinuous non-perfect of non-terminative verbs are synonymous.
Cf. He l a y on his couch thinking and He was lying on his couch thinking, though lay just makes a bare state­ment of the action, whereas was lying stresses its conti­nuity, which makes it more expressive and picturesque.
This is, naturally, not the case with terminative verbs.
Cf. He died and He was dying (but the operation saved him).
Generally speaking, it might be said that the continuous aspect presenting an action in its development retards the narrative making one linger on descriptive scenes.
E. g. They drove the engine in reverse, then forward.
The wheels only dug in deeper. Then the car was rest­ing on the differential again, and the wheels spun freely in the holes they had dug. (Hemingway).
M. Mincoff makes an interest ing observation that in liter­ary narrative the continuous is sometimes used to give the result or consequence of a set of actions suggestive of a cer­tain stability of mood. Such passages are usually introduced by some such phrases as soon after, next moment, three days later, etc.
They quarrelled and he broke their engagement but three days later he was having tea with her at Claridge's. (Black).
With verbs not commonly used in the continuous aspect it emphasizes the transient, temporary, limited nature of the act, ¹ as in I thought I was being unusually considerate. (Wilson).
^{_______________________________________
1} "The shift from 'He has been a loyal subject' to 'He was being a loyal subject' shifts the emphasis from an indefinite state of affairs to a definite action." W. Diver, op. cit., p. 158.

The Future Continuous Non-Perfect

§ 270. If viewed as a language unit, it represents a future action (future tense) viewed in its development (continuous aspect) and not presented as prior to another act or situation (non-perfect order).

E. g. In the afternoon they will be quarrelling in the usual style, no doubt. (Heyer).
When employed with terminative verbs, it expresses a series of repeated acts in the future.