"It is necessary that you do not read that newspaper."
Translation:O gazeteyi okumaman lazım.
It's not ungrammatical, but it's definitely kind of long and awkward. The problem is that they're trying to teach the "okumaman lazım" construction, and the ways that most English speakers would actually express this idea ("You may not read that newspaper." "You can't read that newspaper." "Don't read that newspaper." "You aren't allowed to read that newspaper.") wouldn't be accurate translations of the Turkish sentence. They get the idea across, but the words are way off.
So they've gone with the literal translation, which I think was a good call: it might be awkward, but it avoids the confusion that would come from a non-literal translation, and makes it a lot easier to see what the different words mean.
"It is necessary that you do not read that newspaper." Translation: O gazeteyi okumaman lazım.
okumaman... I really find that confusing. any help as to why the use of that word will be appreciated.
Okumak - (verb) - "Reading."
Oku - Read.
Okuma - (verb) 2nd person, singular - (negative) - "Don't read."
Okumaman - "That you don't read."
That's the best I can do.
I agree that this English sentence is not one which a native speaker would typically use. However, I'm afraid that you are not correct to say that this sentence construction is not valid English. Furthermore, this is misleading for those learning English. It is perfectly acceptable to have stipulations in the negative, i.e.
"You must do X" "You must not do X"
"It is necessary that you do X" "It is necessary that you do not do X"
The use of "must" as above would be the more common and natural sounding way to express the intended sentence.
The use of "important" would not suffice here as something which is "important" is not necessarily required: it's important to remember your umbrella on a rainy day but forgetting it won't get you in trouble with the police.
OK I accept what you are saying. Some things are just not directly translatable from one language to another and this whole section on gerunds is a case in point. I did not say that stipulations in the negative are not possible. My point was that no native speaker of english would use the construction given without qualification. So to translate to 'it is necessary that you do not read that newspaper' is to mislead non-native speakers that this is acceptable english. The response to such a statement from an english speaker would be 'sorry i don't understand, what do you mean?' It is a command and an english speaker would say 'you must not read that newspaper.'