Gender, ser and estar — seen in sentences, not memorised from rules.
Spanish spelling is close to honest: read the letters and you are usually very near the right sound. That is a genuine gift, and it means the first few hundred words come faster than they would in English or French.
The strictness arrives with gender and with verbs. Every noun is masculine or feminine, and the article, the adjective and sometimes the past participle all have to agree. "El agua está muy fría" looks inconsistent — a masculine-looking article with a feminine adjective — and it is one of the small traps a word list will never show you. A sentence will.
Ser and estar both translate as "to be", and choosing wrongly is the mistake that marks a beginner fastest. Roughly: ser for what something is, estar for how or where it is right now. "Ella es una colega fiable" uses ser because it is a lasting quality. "El agua está muy fría" uses estar because it is a current state.
Nobody learns this from the rule. People learn it from meeting the two verbs in hundreds of ordinary sentences until the wrong one starts to sound wrong.
The words in this game are the ones understood across both. There are real differences — vocabulary, the vosotros form, the sound of c and z — but they are far smaller than beginners fear. Learn the shared core first; the regional flavour attaches itself later, wherever you end up using the language.
When a set of Spanish words feels easy, change the language you speak so Spanish becomes the source. That direction is harder and much more useful, because it is the one you need when you are the person doing the talking.