This is the key distinction of the track. An adjective ending in -o changes to -a for feminine. An adjective that does not end in -o does not change at all.
They still take -s for plural either way.
Es caro · Es cara · Los carros son caros · Las máquinas son caras · La máquina es cara
Nouns behave differently from adjectives here. A noun like profesor can take an a to mark a female, even though it does not end in -o.
You do not need a word for a here: he is a professor is just es profesor.
Any Spanish word ending in a consonant takes -es, not just -s.
profesor → profesores · doctor → doctores
Son profesores · Son profesoras · Son doctores · Son doctoras
Es is is: it is, she is, he is, and formal you are. The they are and you guys are form, which as always band together, is son.
El carro es caro · Los carros son caros
English words ending -ive tend to become -ivo. The masculine singular is the form you always start from and build out of.
Spanish does not really have a neuter, only masculine and feminine, but the concept exists. When you are referring to something without a gender, such as a whole situation, you use the masculine singular.
Es adictivo · Es intensivo · Es positivo / Es positiva · Son positivos · Son intuitivos / Son intuitivas · Ella es muy atractiva · Ellos son agresivos · Él es negativo · Son nativos / Son nativas
English double letters that serve no real purpose become single letters in Spanish. Aggressive does not keep its double g, and its double s goes too: agresivo.
rr and nn cross over
Double f, double s, double g: all become single. But connotación keeps its double n, "con-notación".
Positivo has an s, not a z. English positive pulls most speakers towards /pozitivo/.