有關英文句子的問題

My suggestion is:

Last Friday I was doing my homework in the house, and had just finished it when my father arrived at 3:30pm.

---An Attempt to Answer only----
---The past perfect tense-----
---This tense is formed by had+a past participle. It is used
(1) To speak of an action concluded before a certain time in the past Or before the time of the occurrence of another action,denoted by the Simple Past tense) and yet seems continuing into it,
eg as you said:-I had done my HW before(=prior)he (=father) arrived 15:30
eg:-as you said in subsequent to last Friday:-My father arrived at house in subsequent to/subsequently as-----adverb/following I did my HW last Friday.Just as the Present Perfect Tense is used when the resultant state is still "now", so the Past Perfect Tense would be used to refer, at a subsequent time, to this prior action on HW.
eg:-My father arrived at house following I did my HW .

(2)To express duration up to a certain time in the past,
eg:-By Sunday the time father arrived, I had done that HW for 30 hrs..
eg:-Fri.+Sat.=24 hrs.
eg:-930-330=06:00;
Hence 24+6=30 hrs.(=30 hrs the sum of time in past perfect tense)
Hence to express a past condition or supposition with this tense could be understood.