"I need to buy pencils."
Translation:Мне надо купить карандаши.
«Надо» is a bit more colloquial. Also, «надо» can only be used with infinitives, while «ну́жно» can be used with nouns (then it changes it form according to the gender of the noun: ну́жен, нужна́, ну́жно)
I.e. you can replace them when you have an verb (in its inifinitive form):
- Мне на́до/ну́жно купи́ть карандаши́. = I need to buy pencils. (To me, it's neccessary to buy pencils.)
But you can’t replace them when you have a noun:
- Мне ну́жен каранда́ш. 'I need a pencils.' (Can't use на́до here.)
- Мне нужна́ ру́чка. 'I need a pen' (To me, a pen is neccessary.)
- Мне ну́жно молоко́. 'I need milk.' (To me, milk is neccessary.)¹
- Мне нужны́ карандаши́. 'I need pencils.' (To me, pencils are neccessary.)
¹ Most people around me seem to pronounce «ну́жно» with verbs and «нужно́» with neuter nouns, but the dictionary says it's a mistake.
Yes, карандаши́ is the plural form. Singular is каранда́ш.
In this sentence, we have a verb «купи́ть» 'buy', therefore we use «ну́жно» or «на́до». The form of «ну́жно» is agreed with the verb, not with «карандаши́».
If you didn't have the verb, then you'd use «нужны»: «Мне нужны карандаши» 'I need pencils.'
Can I use Покупить?
Thou canst, if thou wishest to sound really old-fashioned. But Duolingo accepts this not.
What is the difference?
- Покупать is the imperfective verb ('to be buying').
- (Its perfective pair покупить is not used in modern Russian, althouth it was possible in the past.)
- Купить is the perfective verb ('to buy').
- (You can't form an imperfective pair for it using the -а- suffix, because купать is already a different verb meaning 'to bathe someone, to wash'.)
Probably it was покупать.
You can't say «Мне нужно покупать карандаши» because it would emphasise the process: you need to be in process of bying pencils during some period of time. This might work in some cases (e.g. when you're buying pencils repeatedly), but in most cases this would sound strange.