No entiendo.
When to use
This phrase is used when you cannot understand spoken or written Spanish. It is polite enough for most situations, though a more formal version exists.
How it works
This is a negative statement using the verb 'entender' (to understand). 'No' negates the verb, and 'entiendo' is the first-person singular present indicative form, meaning 'I understand'.
Object Pronoun
The verb 'entender' can take an object pronoun (e.g., 'No lo entiendo' - I don't understand it). In this basic form, the object is implied by context.
Notes
Use when speech or text is unclear. Neutral everyday wording.