1. If I say, 'Think of the good moments of this day', then all is redundant. Right? So why should I say 'all the'?
I understand what you're asking but I don't like your use of the word 'redundant'. It seems to me that the word
all is probably a way of emphasising that there are lots of good moments. I don't think it's fair to say that that is redundant because it brings a kind of meaning to the utterance without which the utterance would be lacking.
2. So both all the good moments of this day and all good moments of this day are correct, with the meaning changing depending on whether we use the?
Well, they're both grammatically correct, yes, but your question isn't about grammar. The difference between the two sentences is that the first uses a definite article and the second doesn't. That's a difference in meaning.
Care to explain the difference?
Not really. Not yet, at least. Firstly, because Phaedrus has already supplied what I think is an interesting answer. Secondly, because I'm not certain what the sentence is supposed to mean. Thirdly, because I think we need first to separate the meaning of
all from the meaning of
the.
One way of using
all is to express proportional relations. Imagine you're robbing a bank, instructing the clerk to give you:
all (of) the money
half (of) the money
some of the money
none of the money
The meaning of these quantifying phrases expresses a proportional relation of some kind. In other words, these phrases each relate to the totality of the quantity of money. With this in mind, I could interpret your sentence as meaning that you want me to think of 100% of the good moments of the day. You don't want me to think of only some of the good moments of the day. Another interpretation is that the word
all is merely emphatic. Look at this example:
Think of all the people who've died from Covid.
I'm not suggesting here that you think of 100% of coronavirus deaths. I'm merely emphasising that you remember that a lot of people have died.
Which kind of meaning do you think the writer of the sentence actually meant?