Must: يَجِبُ أَنْ + Subjunctive

يَجِبُ أَنْ - "it is necessary that"

يَجِبُ means "it is necessary". It is not "I must": it never changes for the person. The person sits in the verb that follows أَنْ ("that"):

Plain present	After يَجِبُ أَنْ
أَذْهَبُ - I go	يَجِبُ أَنْ أَذْهَبَ - I must go
تَذْهَبُ - you go (m)	يَجِبُ أَنْ تَذْهَبَ - you must go (m)
تَذْهَبينَ - you go (f)	يَجِبُ أَنْ تَذْهَبي - you must go (f)
يَذْهَبُ - he goes	يَجِبُ أَنْ يَذْهَبَ - he must go
تَذْهَبُ - she goes	يَجِبُ أَنْ تَذْهَبَ - she must go
نَذْهَبُ - we go	يَجِبُ أَنْ نَذْهَبَ - we must go
تَذْهَبونَ - you go (pl)	يَجِبُ أَنْ تَذْهَبوا - you must go (pl)
يَذْهَبونَ - they go	يَجِبُ أَنْ يَذْهَبوا - they must go

What أَنْ does to the verb

After أَنْ the verb is in the subjunctive (المُضارِعُ المَنْصوبُ): the final -u becomes -a (تَذْهَبُ - تَذْهَبَ), and the endings -īna and -ūna lose their ن (تَذْهَبينَ - تَذْهَبي, تَذْهَبونَ - تَذْهَبوا, written with a silent alif). In speech the final vowel is often dropped, so the change matters most in reading and writing.

English puts a bare infinitive after "must" (I must go). Arabic has no infinitive: literally it says "it is necessary that I go". The same أَنْ + subjunctive follows أُريدُ أَنْ (I want to) and يُمْكِنُ أَنْ (it is possible to).

At A1, build the sentence with أَنْ + a verb with its person. On signs and in instructions you will also meet a second pattern, يَجِبُ + a verbal noun: يَجِبُ ارْتِداءُ الكِمامَةِ - masks must be worn.

Naming who must: عَلى

Put the person on the preposition عَلى, "on": يَجِبُ عَلَيَّ أَنْ أَذْهَبَ, or just عَلَيَّ أَنْ أَذْهَبَ - it is on me to go, I have to go.

عَلَيَّ - on me	عَلَيْنا - on us
عَلَيْكَ - on you (m)	عَلَيْكُمْ - on you (pl)
عَلَيْكِ - on you (f)	عَلَيْهِمْ - on them
عَلَيْهِ - on him	عَلَيْها - on her

The verb after أَنْ still carries its person: عَلَيْكِ أَنْ تَسْتَريحي - you (f) have to rest.

Must not, don't have to

English separates "you mustn't" from "you don't have to", and so does careful Arabic, by where لا stands:

يَجِبُ أَلّا تَعْمَلَ اليَوْمَ - you mustn't work today. The negation comes after أَنْ; أَنْ + لا is written أَلّا (also seen as أَنْ لا).
لا يَجِبُ أَنْ تَعْمَلَ اليَوْمَ - you don't have to work today. The negation comes before يَجِبُ.
لَيْسَ عَلَيْكَ أَنْ تَدْفَعَ - you don't have to pay.

A warning about real Arabic: that is the careful, textbook use. In newspapers, on signs and in everyday speech لا يَجِبُ أَنْ is very often used to mean "must not". So when you write a prohibition, use يَجِبُ أَلّا, which nobody can misread; when you mean "you don't have to", لَيْسَ عَلَيْكَ أَنْ says it with no ambiguity. When you read لا يَجِبُ أَنْ, let the context decide.

1. Changing يَجِبُ for the person: *أَجِبُ أَنْ أَذْهَبَ. يَجِبُ does not change for the person. 2. No person on the verb, as if it were an infinitive: *يَجِبُ أَنْ ذَهَب. Say يَجِبُ أَنْ أَذْهَبَ / تَذْهَبَ. 3. Keeping the ن after أَنْ: *أَنْ تَذْهَبينَ, *أَنْ تَذْهَبونَ. Say أَنْ تَذْهَبي, أَنْ تَذْهَبوا. 4. لا يَجِبُ أَنْ for "must not": in careful usage it means "don't have to"; the press and everyday speech often use it for "must not", but it is ambiguous. For "must not" say يَجِبُ أَلّا. English speakers: after "must" English uses a bare infinitive; Arabic needs "that" + a verb with a person, so "I must go" is "it is necessary that I go". The good news: careful Arabic draws the same line between "mustn't" and "don't have to" that English does.

Frequently asked questions

When is Must: يَجِبُ أَنْ + Subjunctive used?

يَجِبُ means "it is necessary". It is not "I must": it never changes for the person. The person sits in the verb that follows أَنْ ("that"):

How is Must: يَجِبُ أَنْ + Subjunctive formed?

After أَنْ the verb is in the subjunctive ( المُضارِعُ المَنْصوبُ ): the final -u becomes -a ( تَذْهَبُ - تَذْهَبَ ), and the endings -īna and -ūna lose their ن ( تَذْهَبينَ - تَذْهَبي , تَذْهَبونَ - تَذْهَبوا , written with a silent alif). In speech the final vowel is often dropped, so the change matters most in reading and writing. English puts a bare infinitive after "must" (I must go). Arabic has no infinitive: literally it says "it is necessary that I go". The same أَنْ + subjunctive follows أُريدُ أَنْ (I want to) and يُمْكِنُ أَنْ (it is possible to). At A1, build the sentence with أَنْ + a verb with its person.

What are common mistakes with Must: يَجِبُ أَنْ + Subjunctive?

1. Changing يَجِبُ for the person: *أَجِبُ أَنْ أَذْهَبَ. يَجِبُ does not change for the person. 2. No person on the verb, as if it were an infinitive: *يَجِبُ أَنْ ذَهَب. Say يَجِبُ أَنْ أَذْهَبَ / تَذْهَبَ. 3. Keeping the ن after أَنْ: *أَنْ تَذْهَبينَ, *أَنْ تَذْهَبونَ. Say أَنْ تَذْهَبي, أَنْ تَذْهَبوا. 4. لا يَجِبُ أَنْ for "must not": in careful usage it means "don't have to"; the press and everyday speech often use it for "must not", but it is ambiguous. For "must not" say يَجِبُ أَلّا. English speakers: after "must" English uses a bare infinitive; Arabic needs "that" + a verb with a person, so "I must go" is "it is necessary that I go".

Sources

W. Wright, A Grammar of the Arabic Language, Vol. II (3rd ed., rev. W. Robertson Smith and M. J. de Goeje) — Cambridge University Press, 1898

§ 15 (a) (pp. 24-25): the subjunctive is governed by أَنْ "after verbs which express ... duty, ... necessity, permission", and أَلّا / أَنْ لا means "that not". This is the rule behind يَجِبُ أَنْ تَذْهَبَ and يَجِبُ أَلّا تَذْهَبَ.

W. Wright, A Grammar of the Arabic Language, Vol. II (3rd ed., rev. W. Robertson Smith and M. J. de Goeje) — Cambridge University Press, 1898

§ 59 (c) (p. 169): عَلى is used "of a debt which one owes, and a duty which is incumbent upon one", with the example عَلَيْكَ أَنْ تَفْعَلَ هٰذا rendered "it is incumbent upon thee to do this, thou must do this".

A Reference Grammar of Modern Standard Arabic (Karin C. Ryding) — Cambridge University Press, 2005

Chapter 21, "Verb inflection: a summary" (pp. 438-454), is the modern reference summary of MSA verb inflection. Cited from the publisher's catalogue record; the chapter text was not consulted.