If you include the word "more" in a sentence without "than" in spoken English, would it be valid? Example: a talk about steps how to resolve environmental issues.
I'll get into them in more detail later as I progress through the talk.
Some say that do not use comparative adjectives such as, more, better and so on without "than" in a sentence/clause, because it would be ungrammatical. However, in the above sentence, it sounds right. Is this an exception?
This sentence is completely fine. It would seem weird to squeeze some comparison in unless it isn't obvious from context.
I'll get into them in more detail (than I am now?) later as I progress through the talk.
I would like more drink (than I have now?), please.
With the than just sounds unnatural, especially in the second sentence.
I would, however, like to correct your preposition usage. While into is fine, I would use just to here:
I'll get to them in more detail later as I progress through the talk.
English, in practice, often drops comparators (that's not, as far as I'm aware, a grammatical term; I'm borrowing its use in various academic disciplines, public policy and so on, for "the thing to which something is to be compared). If the comparator is obvious from context, you don't need to say it - and often saying it comes across as wrong.
"Why do you think A's more attractive than B?"
"He's taller."
Obviously, this is saying that A is taller than B, but including that would be unnecessary and can make it seem strange.
Similarly, when making an argument, presenting a talk, and so on, you will often make comparisons that seem, just from the words used at that time, to be missing their comparator, but everyone knows what it means, so it's unnecessary. Adding the comparator would break the flow, and leave everyone wondering "why did they bother to say that?".
I would say the general rule is not to use "than" expressions and other comparators unless necessary. We'd get horribly bogged down if we couldn't say "I'm stronger now", which is the same grammatically as any "more" expression. "I'm stronger now than I was" is unwieldy (though occasionally preferable for aesthetic reasons), and even that is actually missing a location in time. "I'm stronger now than I was a year ago" should only be said if there's some significance to "a year ago" in the context, and in fact would often end up being shortened to "I'm stronger now than a year ago", which if you try to parse it makes no sense unless you add an unwritten "I was".
However, if you use language of comparison and the comparator isn't obvious from context, it will read as wrong.
