Learn Swedish: Läraren ger råd till eleverna. - The teacher gives advice to the pupils.

Unlike English where 'advice' is always uncountable, in Swedish it is a regular countable noun: ett råd means 'a piece of advice'. Because it is an ett word ending in a consonant, its plural is exactly the same: råd. Here, it is used in the plural indefinite form to mean 'pieces of advice' in general.

Could I say this without the preposition till?

Yes! Just like in English ('The teacher gives the pupils advice'), you can drop till by changing the word order. You simply put the indirect object (eleverna) right after the verb, before the direct object: Läraren ger eleverna råd.

The word for teacher is en lärare. Why is the definite form läraren instead of lärareen?

For en words that already end in a vowel, you just add an -n to make them definite, avoiding an awkward double vowel. This is standard for nouns ending in -are. Watch out for the definite plural, though, which drops the -e- entirely: lärarna (the teachers).

Ger is an irregular verb. What are its past tense forms?