Does this sentence indicate something about completion of an action or something in the past?
'verb + 了' indicate the verb is completed, but it doesn't indicate any tense, It can be in the past or in the future.
昨天我(吃了)就睡 - I slept after I ate yesterday
現在我(吃了)又要去睡 - now I've eaten, I have to sleep again
明天我(吃了)還是要去睡 - I will still sleep after I've eaten tomorrow.
In all three sentences, 了 only indicate the action 吃 is completed.
'If' is implied.
如果(if) 别人(people) 用了 (used) 我们的赠品 (our gifts)，觉得好 (consider it is good)
别人(people) 用了 (used) 我们的赠品 (our gifts)，如果(if they) 觉得好 (consider it is good)