How to say I need to study in Spanish

Necesito estudiar.

neh-seh-SEE-toh ess-too-DYAR

Necesito is "I need"; estudiar is the infinitive "to study". There is no word for "I" because the -o ending already says who is speaking.

What people actually say: Tengo que estudiar. Literally "I have to study", and far more common in speech than necesito estudiar. If you want to sound like a student rather than a textbook, this is the one to use.

The phrases around it

English	Spanish	Note
I need to study.	Necesito estudiar.	The literal translation of the English.
I have to study.	Tengo que estudiar.	What a Spanish speaker actually says.
I must study.	Debo estudiar.	Stronger, and a little formal.
I need to study more.	Necesito estudiar más.	Más goes after the verb, not before it.
I have to study for an exam.	Tengo que estudiar para un examen.	Para, not por.
I need to study Spanish.	Necesito estudiar español.	Languages are lower case in Spanish.
I am studying.	Estoy estudiando.	Estar plus the gerund — never ser.
I studied all night.	Estudié toda la noche.	Preterite: one finished stretch of time.
I cannot, I have to study.	No puedo, tengo que estudiar.	The full sentence you are looking for.
I have an exam tomorrow.	Tengo un examen mañana.	Mañana is both "tomorrow" and "morning".

Necesitar takes the infinitive, not a conjugated verb

Necesito estudiar, never necesito estudio. The same shape works for anything you need to do: necesito dormir, necesito comer, necesito hablar contigo. Tener que and deber behave the same way — the second verb stays in its dictionary form.

Tengo que is the everyday one

Necesito estudiar is correct and will be understood everywhere, but it lands slightly closer to "I require studying" than to what you meant. Spanish speakers reach for tengo que for obligation and save necesitar for a genuine need. Hay que estudiar is the impersonal version: "one has to study".

Estudiar vs. repasar

Estudiar covers learning something new. Repasar is going back over material you have already seen — revising. Tengo que repasar para el examen is what you say the night before, and it is the more precise verb for what most people mean when they say they need to study.