Questions & Answers about Tengo cierta duda sobre la lección.
In Spanish, many things that are states or feelings in English (using to be or to feel) are expressed with tener (to have).
Examples:
- Tengo hambre. – I am hungry. (literally: I have hunger.)
- Tengo frío. – I am cold.
- Tengo miedo. – I am afraid.
- Tengo una duda. – I have a question / doubt.
So Tengo cierta duda… literally means I have a certain doubt…, but functionally it’s like saying I’m a bit unsure… or I have a question….
Using estoy dudando sobre la lección (I am doubting the lesson) would sound more like you don’t believe the lesson is true, which is usually not what a student wants to say.