Present perfect with simple past

Question

I have a question regarding using present perfect and simple past within the same sentence. Does it make sense to say the following sentences?

"He has agreed to help me, so I did not do my homework."

"She has suddenly decided to go back, so I decided to go alone."

And I'm curious about which tense comes first. When you use present perfect and simple past in the same sentence, did simple past take place first and is present perfect more recent? Or does it depend on context?

Another question:

I read the following sentence in a news article: "[she] quietly slipped away today from the hospital where she has been staying since being freed..." It seems like this sentence is using simple past and present perfect at the same time. Is this a different case? I know that tenses have to match, but are there exceptions?

I also saw this sentence: "If you have just registered, you received your CURRENT bill via USPS."

Answer 1

The two sentences violate the sequence of tenses. They should be:

"He has agreed to help me, so I will not do my homework."
"She has suddenly decided to go back, so I have decided to go alone."

First comes the action in the first clause in both sentences, then the decision in the second clause in both sentences. You cannot go from the present prefect to the simple past that way. The sequence is past perfect to simple past or simple past to simple past:

"He had agreed to help me, so I did not do my homework.
"She had suddenly decided to go back, so I decided to go alone."

Another way of writing these two sentences is:

"Because he has agreed to help me, I will not do my homework."
"Because she suddenly decided to go back, I have decided to go alone."

In both cases, there's a cause-effect relationship stated.

Here's another link

Answer 2

They violate the sequence of tenses because the first action ("he has agreed help me") is in the present tense (perfect aspect) and the second ("so I did not do my homework") is in the past tense. The same is true for your second sentence. In both sentences, the first action must be either in the simple past or the past perfect if the second action is in the simple past: that's the sequence of tenses rule. If the first action is in the present perfect, the second action can be in the future ("will not do"). The reason I mentioned the cause-effect relationship is that it puts the cause (first action) first and the effect (second action) second, which is the sequence of events. The sequence of tenses must reflect the sequence of events. This is true whether or not there is a cause-effect relationship between two events. All that's important is that one event followed another in time: that establishes a sequence of events that must be reflected in the sequence of tenses.