НАДО, НУЖНО, МОЖНО – Perfective or Imperfective? That’s the question!

According to young RFL teachers, one of the most difficult topics for learners is choosing verb aspect in the infinitive, especially after the modal expressions “надо”, “нужно”, “можно” and “должен” (corresponding to “need”, “necessary”, “possible”, and “must”). So where should teaching this topic begin?

In my view, teaching should begin with a review of the basic meanings of the imperfective and perfective aspects. The perfective aspect presents an event as bounded or complete, often focusing on its result, beginning or endpoint. One related meaning is that of a single or one-off occurrence. It is also worth noting that perfective verbs have only two tense forms – past and future – which is consistent with the fact that a completed result cannot be presented as unfolding in the present. The infinitive itself lacks tense, but the temporal context should still be taken into account when choosing an infinitive form. 
The imperfective aspect presents an action as ongoing or unbounded, without focusing on an endpoint. Imperfective verbs have three tenses: present, past and future.

Secondly, the teacher should know that after “надо” (“need”), “нужно” (“necessary”), “можно” (“possible”), “должен” (“must”), and “нельзя” (“impossible/not allowed”), infinitives of both aspects can be used, depending on the meaning.
Any grammar topic is easier to understand when it is embedded in a communicative context. Suitable situations for this topic include “Request”, “Advice” and “Recommendation”. Role-play dialogues may use scenarios such as “Parents and Children”, “Teacher and Students”, “Doctor and Patient” and “Boss and Subordinate”. Comics or story-based pictures connected with these situations can also be useful.

The main distinction here is between a single and a repeated action, and between a bounded result and an ongoing process. Thus, in “Нужно пить воду каждый день” (“You need to drink water every day”), the imperfective “пить” is used for a repeated action; in “Нужно выпить воды” (“You need to drink some water”), the perfective “выпить” presents a single specific action. The same principle applies to the other modal constructions discussed above. This contrast can already be introduced at A2 level.

At higher levels, however, further difficulties arise. For instance, what is the difference between “Нужно ложиться спать” and “Нужно лечь спать” (both can be translated approximately as “You need to go to bed”)? The imperfective “ложиться” can present going to bed as a general, repeated or currently relevant course of action, whereas the perfective “лечь” typically refers to a specific future occurrence, often in the near future. Moreover, the perfective verb points to a specific intended result rather than merely the general desirability of an action.

When these words are used in negative constructions with “не” – “не надо”, “не нужно”, “не должен” – only the imperfective infinitive is used. For example, “Я не должен читать эту книгу” (“I shouldn’t read this book”) uses “читать” rather than “прочитать”.
With «нельзя», the situation is slightly different. If the speaker means that an action is forbidden or not considered acceptable, the imperfective is used: “В комнату нельзя входить, там спит мама” (“You mustn’t go into the room; Mum is sleeping there”). If the meaning is physical impossibility, the perfective is used: “В комнату нельзя войти, у нас нет ключа” (“We can’t get into the room – we don’t have the key”). The contrast is logical: in the first case, the action is treated as prohibited or not customary; in the second, it cannot be completed in the present situation. There are, however, exceptions. In the Russian language, some verbs are imperfective-only, for example “дышать” (“to breathe”). With such verbs, “нельзя” may express both prohibition and physical impossibility, while the infinitive remains imperfective: «Нельзя дышать под водой» (“You can’t breathe underwater”).

It should also be noted that these words normally take a dative experiencer, whereas “должен” agrees with a nominative subject (compare: Мне нужно помыть посуду – need to wash the dishes; Я должен помыть посуду – I have to wash the dishes). 
Teachers should also point out that “нужно” and “надо” are synonyms, although “надо” is more characteristic of colloquial speech.

There are also verbs and constructions that characteristically select either the imperfective or the perfective infinitive. The imperfective is commonly used after:

1. Verbs denoting the beginning, continuation or end of an action, as well as the verb “стать” (“to become”): “заканчивать”/”закончить” (“to finish”), “начинать”/”начать” (“to begin”), “продолжать”/”продолжить” (“to continue”),”переставать” / ”перестать” (“to stop”), etc. 
2. Verbs such as “уставать”/”устать” (“to get tired”), “надоедать/надоесть” (“to become tiresome” / “to get tired of doing something”), and “запрещаться” (“to be prohibited”).
3. Verbs such as “учиться”/”научиться”/”разучиться”» (“to learn” / “learn how” / “lose the ability to”), “привыкать”/”привыкнуть”» (“to get used to”), “отвыкать”/”отвыкнуть”» (“to get out of the habit of”), and “любить”/”полюбить” (“to like” / “come to like”).
4. Negative constructions – “не следует” (“shouldn't”), “незачем” (“there's no need”), “лучше не” (“better not”), “не советую” (“I don't advise”), “не хочется” (“I don't want to”), “достаточно” (“enough”), “хватит” (“enough”), “вредно” (“harmful”), etc.

It is worth noting that all these verbs and constructions are either associated with negation, denote a process, or express “fatigue” from repetition of an action. The perfective is commonly used after verbs such as “суметь” (“to manage to”), “забыть” (“to forget to”), “успеть” (“to manage” /” have time to”) and “удаваться/удаться” (“to succeed in managing to do something”), because these meanings focus on achieving a result.

With other verbs — for example, “хотеть” (“to want”), “просить” (“to ask”) and “советовать” (“to advise”) – the choice of aspect depends on the speaker’s communicative intention. Thus, “Я советую вам покупать продукты в этом супермаркете” recommends shopping there regularly, whereas “Я советую вам купить продукты в этом супермаркете” recommends a specific purchase on a particular occasion.

Of course, practice is essential. Exercises on choosing aspect in the infinitive can be found in such manuals as “Улучшим наш русский” (“Let’s Improve Our Russian”), “A Living Russian Grammar” and “Чистая грамматика” (“Pure Grammar”). Various tests are available in the TORFL-2 and TORFL-3 preparation books “Тесты, тесты, тесты” (“Tests, Tests, Tests”).

Author: Yulia Amlinskaya, teacher of Russian as a Foreign Language and administrator of the “RFL Teachers” professional community.