"I have to study."
Translation:Tengo que estudiar.
It's been five years and you almost certainly know this now, but for anyone else first reading this thread:
No two languages ever map word-for-word. In English, we indicate obligation by saying that we
have to do something; Spanish says that we
have that do something: Tengo que hacerlo.
If it helps, think of it as saying, "this is something that I have to do".
Here is an explanation I found: '"Tengo que" is the informal way of saying "Deber." Comparing the two phrases to English can be confusing because in English, "Have to" is obligation and "Should" is advisory; they have different meanings, whereas in Spanish, "Deber" can mean either, "Should" or "Have to." "Deber" can also mean probability: "Por como está vestido, él ha de ser soldado." "From how he is dressed, he must be (probably is) a soldier."'
Also of note, Spanish speakers seem to use 'deber' more as 'must' rather than 'should' implying greater obligation than the less formal 'tengo que' (literally means 'to have to') which is use more like 'should' is in English.