That's it. Нужно doesn't change when it is followed by the infinitive. However, when you're talking about an object you need, the form depends on the gender of this object. Мне нужЕН друг. (m) Мне нужнА книга. (f) Мне нужнО время. (n) Мне нужнЫ друзья. (pl) With должен it's a little bit different. What counts is the gender of the subject, i.e. the word which normally precede должен (except in questions) Он должЕН работать. Она должнА работать. Оно должнО работать (радио, например). Они должнЫ работать. Я спрашиваю, должен ли человек работать? (in questions the subject is put after должен) To sum up, with нужно you have to pay attention to the word which is following it (an object), with должен - to the word preceding it (the subject).

1. what is the difference between нужно , надо and должен . in english "have to" means necessity (вынужденность,необходимость) in weak degree, "must" In duty degree. In Russian degree "надо" is meant from a context degree "нужно" weaker then "надо" in a case "have to" must - as a public rule have to - as a personal rule, moral standards of the certain person need - requirement of the person, both moral, and physical. Мне нужно сходить в магазин. (Mother says when it wasn't vitalt to go here to her ) Вам надо доделать этот отчет. (The boss says when he gave the strict commission to the subordinate) The sensible-word-formation dictionary: Нужно - An estimation something as necessary that it is necessary to make/To have. Надо - The colloquial - нужно, необходимо. 2. ".....is this right?" yes, you are right. 3. sorry for errors which meet in comments. A question is really very interesting and it's difficult to translate correctly all nuances. good luck! It is glad, if i could help you

The margin between them is very slim when they refer to a verb. мне надо идти домой = мне нужно идти домой = = я должен идти домой = I have to \ should go home However должен cannot refer to a noun(in that case должен= to owe), while нужно\надо can. Example: мне надо\нужнА коробку\а (I need a box). Я должен эту коробку кое-кому = I owe this box to someone (here должен refers to the noun the box = коробка) Я должен пнуть эту коробку = I have to kick this box (here должен refered to the verb to kick = пнуть) And you should keep in mind that when нужен\надо refer to a noun they somehow change the endings differently as you can see in the example above.

Должен(обязан) = must ----- Нужно = need ... "I need you because I love you" "Ты нужна мне потому, что я люблю тебя" ... "You ought to have it" "Вам нужно(следует) иметь это" ... "I have to go 'cause I wanna do something" "Мне нужно идти потому, что я хочу сделать что-то" [In any case when "I want something"] ------ Надо = "forced" need ... "I need it because I will die without that" "Мне надо это потому, что я умру без этого" [In any case when "somebody(something) wants me to do something" - but i'm not forced] ----- Русскоязычные всё ещё спорят о различности этих слов :-)

нужно и надо - it's the same должен - it's should or must in different cases/