我吃了早饭就去了教室。 去了 + somewhere. 去 + do something + 了. v + 了 means done.

1_他感冒了，昨天我去了看他。 correct: 他感冒了，昨天我去看了他。/他感冒了，昨天我去看他了。 reason：”我去了看他“sounds like future tense，so it can't be used in a past tense(昨天)。you can regard "去看”as a verb，verb+了 means here a past action. 2_昨天我吃了早饭就去教室。 correct:昨天我吃了早饭就去教室了。 reason:吃了早饭就去教室 means a future action，I am going to have breakfast and then go to classroom. So it can't be used in a past tense(昨天)。Here the action should have finished. 3_那天他下了课就来我这儿。 correct: 那天他下了课就来我这儿了。 reason: same reason as the second sentence

i can't explain why these sentences are wrong,but if let me say these three sentence in chinese.i would say"1.他感冒了，昨天我去看他了 2，昨天我吃了早饭就去教室了 3，那天他下来课就来我这儿了