Two Ways to Express Purpose

In Hungarian, expressing 'why' you do something follows specific rules based on the type of verb and the subject.

1. The Purpose Infinitive
The infinitive is the most natural way to express purpose when the main verb involves motion (megy - go, jön - come, elindul - set off, megáll - stop), transfer or provision (ad - give, hoz - bring, visz - take), calling or contacting (hív - call, telefonál - telephone), or staying (marad - stay). Note that purpose infinitives always remain in their bare -ni form and do not take personal infinitive endings. For example, it is always Elmentem aludni, never Elmentem aludnom.
Example: Elmentem aludni. (I went [in order] to sleep.)
Example: Felhívtam megkérdezni... (I called to ask...)

2. The Hogy-Clause (General Purpose)
For most other verbs that do not involve motion, transfer, staying, or calling, you generally use hogy followed by the subjunctive mood. Note that motion verbs can also use a hogy-clause, particularly when the main clause uses the correlative word azért (for that purpose) as an anchor to emphasize the purpose.
Example: Azért edzek, hogy erős legyek. (I train [in order] to be strong.)
Example: Azért mentem, hogy segítsek. (I went [for the purpose] to help.)

3. Different Subjects
If you do something so that someone else does something, you generally use the hogy structure. However, certain verbs of sending, transfer, or provision (like küld, ad, hoz, visz) allow an object plus an infinitive, even with different subjects.
Example: Pénzt adok, hogy vegyél kenyeret. (I give money so that you buy bread.)
Example: Adok a kutyának enni. (I give the dog [something] to eat.)
Example: Elküldtem őt aludni. (I sent him to sleep.)

4. Negative Purpose
To express 'so that... not' or 'lest', use nehogy instead of hogy, or use hogy ne.
Example: Siettem, nehogy elkéssek. (I hurried so I wouldn't be late.)

5. Sequence of Tenses
Unlike English, Hungarian does not backshift tenses in purpose clauses. Even if the main verb is in the past tense, the hogy-clause must remain in the present subjunctive. Do not use past tense or conditional forms here (e.g., Siettem, nehogy elkéssek uses the present subjunctive elkéssek despite the past tense siettem).

Key Morphological Notes:
In the hogy-clause, the verb must be in the subjunctive. This usually involves the -j- marker. Remember that the subjunctive verb must still agree in definiteness with its object, choosing between the indefinite and definite conjugation (e.g., hogy vegyél kenyeret [indefinite] vs. hogy megvedd a kenyeret [definite]). In a neutral, affirmative purpose clause, verbal prefixes stay attached to the verb (e.g., Azért írtam, hogy megkérdezzem). Crucially, if the clause is negated with hogy ne, it undergoes prefix separation, moving immediately behind the verb (e.g., hogy ne késsen el). However, if negated with nehogy, the prefix stays attached (e.g., nehogy elkéssek). Always place a comma before hogy or nehogy.