I think it very likely that your English teacher was giving you ‘baby rules’: the sort of very broad rules you give a child (or learner) to prevent it from hurting itself before it is old enough (or knowledgeable enough) to understand subtle qualifications.
In the case of too, your teacher was probably ‘protecting’ you from a very common learner’s error: using too as a simple intensive, equivalent to very.
The ‘grown-up’ rule is that although it is quite true that too almost always involves a comparison of the sort you have in too hot to go out, this does not mean that the comparison must actually be expressed in every sentence in which you use too. The benchmark against which you are measuring may be entirely clear from the context.
Do you want to go for a walk?
No, thanks, it’s too hot.
In the case of so, your teacher was probably trying to keep you from using intensive so in an inappropriate register. This use of so is colloquial and almost never found in formal or semi-formal registers such as you are typically called upon to employ in school essays. But in conversation (or dialogue, if you are writing fiction or a play or a script) it’s perfectly acceptable:
It is so hot today!
Such rules are pedagogically useful: they keep you focused on what you must learn and keep you from being distracted by all the million-and-one-things you might learn. But in language (as in pretty much everything else), once you reach a certain level of competence you may safely ignore classroom rules and instead follow the practice of writers and speakers you admire.
As was pointed out on that other question you linked, this is a surprisingly tricky sentence!
Let's slowly build it up.
It will take someone else.
What is "it"? Stopping Voldemort again. Implied but never directly said. As for "take", we could say "need" instead, just to be slightly clearer.
[Stopping Voldemort again] will [need] someone else.
This isn't "someone else" as in "we need a different person", but as in "we need other people prepared to do what you did, Harry". When will they be needed? "Next time" (that is, the next time that Voldemort tries to return).
[Stopping Voldemort again] will [need] someone else who is prepared to [do the same as Harry] next time.
Now, Dumbledore is saying that this someone else doesn't have to do much. Hence, it will "merely" (or "only") take someone else. This may sound like it's demeaning Harry's efforts, but it's meant to be reassuring Harry: standing up to Voldemort is not very difficult or unlikely after all, and it doesn't take someone extra-special to do it.
[Stopping Voldemort again] will merely [need] someone else who is prepared to [do the same as Harry] next time.
What, exactly, did Harry do that Dumbledore says they need other people to do (or be prepared to do)? "Fight a losing battle"--or what seems like a losing battle. (Dumbledore omits the word "like", but I'll leave it in for this one example.)
[Stopping Voldemort again] will merely [need] someone else who is prepared to fight a losing battle next time.
[Stopping Voldemort again] will merely [need] someone else who is prepared to fight what seems [like] a losing battle next time.
And now we just replace the bracketed bits with the different wording Dumbledore uses, and we have the sentence (okay, part of a sentence) that you bolded.
It will merely take someone else who is prepared to fight what seems a losing battle next time.
Best Answer
This goes back to two related but very different meanings of the verb confuse
One meaning of confuse takes two objects, call them A and B. To confuse A and B means to “mix them up”, to mistake A for B and B for A.
In this sense we also say confuse A with B.
When you turn confuse into the noun confusion, you should use of to designate its two objects.
The other meaning of confuse is to “bewilder” or “puzzle” someone—usually a person, but an animal can be confused as well.
When you say “I am confused”, you are using this sense in the passive voice: something confuses you. We have two ways of designating that ‘something’: either with by or with about.
But we never use of with confuse or confusion in this sense:
When you use the noun confusion on ELL, chances are that you are confused about something. Occasionally you will be asking how to avoid confusion of one thing with another. And every once in a while an Answerer will point out that confusion is created by a speaker or writer or teacher:
ADDED:
The question you have added addresses a different verb, which uses prepositions quite differently. (This is usually the case; you can almost never generalize from the use of a preposition in one context to its use in other contexts.)
Think, again, has two different meanings. One is to “examine a matter mentally”; in this sense, the matter you are examining may be expressed with either of or about.
The other meaning of think is to “hold an opinion concerning something”. In a question, we use either about or of to designate the something:
But in statements we use of, complemented by an as phrase; or no preposition complemented by a nominal or adjective phrase; or a full clause, finite or non-finite. We never use about.
∗ marks a usage as unacceptable