Use of SINCE, FOR and AGO. Ago uses the PAST SIMPLE (measuring the time from now to a point in the past). For (a period of time till now) and since (from a definite point till now) use the PRESENT PERFECT (have/has plus past participle).
When the time of one action is defined in relation to a second action, the second is placed in the PAST CONTINUOUS. Formed with the past of to be (was/were) + the verb stem plus "ing".
When the time of a past action is defined in relation to another action then the one which is happening in the background should use the PAST CONTINUOUS. We are not interested in the time of its completion - i.e.
......................was crossing.......................... ......................was (still) crossing
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when I met him
If the actions are consecutive use the SIMPLE PAST. The same with habitual actions in the past. The PAST CONTINUOUS is used when we are not interested in the completion of a past action, simply that it was happening when something else happened.
Note that is is possible to have two present continuous verbs following each other.
We (enter) Port Said harbour when I (come) on deck. As soon as our ship (come) near enough, a large number of boats (set out) from the shore. We could see that they (bring) money-changers, guides, and men selling all the wonderful things of the East. While I (leave) the ship I (notice) that the rest of the passengers (argue) about the prices in loud voices. I (walk) about for an hour and (watch) the gay street life; men (sell) strange fruits and vegetables, and tradesmen (mend) shoes or (make) carpets in their little shops. I (return) to my ship as the sun (set); the 'market' on the ship still (continue). An old man who (sit) on the deck (offer) me a beautiful Persian rug for only £20. I (talk) hard for ten minutes, and just as the ship (go) I (buy) it for £2. As I (go) to my cabin I (see) a sailor with a rug like mine. I (ask) him the price. "Fifty pence" he said, "but I paid too much for it". A man that I (talk) to just now only (pay) twenty-five pence. As I (undress) that night I (notice) a little piece of cloth which (hang) from one corner of my rug. On it were the words "Made in Manchester".
The afternoon sun (get) low as the Rat (scull) gently homewards in a dreamy mood, murmuring poetry-things over to himself, and not paying much attention to Mole. But Mole (be) very full of lunch and self-satisfaction and pride, and already quite at home in a boat - so he thought - and (get) a bit restless besides; and presently he (say) "Ratty"! Please, I want to row now!". The Rat (shake) his head with a smile. But he (begin) to feel more and more jealous of Rat, sculling so strongly and so easily along, and his pride (begin) to whisper that he (can) do it every bit as well. He (jump) up and (seize) the sculls so suddenly that the Rat, who (gaze) out over the water and (say) more poetry-things to himself, was taken by surprise and (fall) backwards off his sear with his legs in the air for the second time, while the triumphant Mole (take) his place and (grab) the sculls with entire confidence.
The gypsies (see) at once that she (be) a little lady, and were prepared to treat her accordingly. There was a group round the fire when they (reach) it. An old gypsy women (sit) on the ground, occasionally poking a skewer into the round kettle that (sent) forth an odorous steam; two small shock-headed children (lie) prone and (rest) their elbows something like small sphinxes; and a placid donkey (bend) his hear over a tall girl, who (scratch) his nose and (indulge) him with a bite of excellent stolen hay. The slanting sunlight (fell) kindly upon them, and the scene (be) really very pretty, Maggie (think), only she (hope) they would soon set out the tea-cups. It (be) a little confusing, though, that the young women (begin) to speak to the old one in a language which Maggie (not understand), while the tall girl, who (feed) the donkey, (sit up) and (stare) at her without offering any salutation.