Hablar y escuchar

Entender and comprender both mean to understand, and the difference is real if slight.

Entender is grasping what was said. No te entiendo is I did not catch that. Comprender is understanding in the deeper sense, taking in the whole of something: comprendo tu situación.

In practice entender is far commoner and covers both. Comprender sounds more considered, and using it about a sentence someone just mumbled would be faintly grand.

oir and escuchar

This pair maps cleanly onto English hear and listen, which makes it one of the easier distinctions in the language.

Oír is passive. Sound arrives at you. Escuchar is deliberate. You direct attention at it. Te oigo pero no te escucho is a sentence with a clear and slightly cutting meaning.

Oír is also wildly irregular - oigo, oyes, oye - and worth drilling on its own.

lengua, lenguaje, idioma

Three words for language and they divide the territory differently.

Idioma is a specific language: Spanish, Japanese, Welsh. It is masculine despite the -a, because it came from Greek, along with problema, sistema and tema.

Lengua is a language too, and also a tongue. Lengua materna is mother tongue.

Lenguaje is language as a faculty or a style: the language of law, body language, programming languages. Never a specific national language.

Ask someone ¿cuántas lenguas hablas? or ¿cuántos idiomas hablas? and both work. ¿Cuántos lenguajes hablas? will get you a puzzled look, or a conversation about Python.

The verbs of saying

Decir is to say and is irregular in almost every direction. Contar is to tell, and also to count, which is the same metaphor English uses in recount.

Preguntar asks a question. Pedir asks for a thing. Confusing them is the classic error, and it is worth fixing early: pregunté un café is not a coffee order, it is a question about coffee.