Nouns
Noun gender, and why it decides the words around it
Every noun is masculine or feminine, and the words next to it have to agree.
Nouns ending in o are usually masculine and take el and un. Nouns ending in a are usually feminine and take la and una. Adjectives and articles both change to match, which is why it is buenos días but buenas tardes.
Examples
el libro rojothe red bookmasculine throughout
la mesa rojathe red tablethe adjective changes too
el agua fríathe cold waterfeminine, but takes el to avoid two a sounds
el problemathe problemone of a small group of a-ending masculine nouns from Greek
The mistake this causes
Learning nouns without their article. Store la mesa, not mesa, and the gender arrives free.