Presente de indicativo

SER and ESTAR both mean "to be" — NOT interchangeable!
Ser = "D-O-C-T-O-R" for Description, Origin, Characteristic, Time, Occupation, Relationship.
Estar = "P-L-A-C-E" for Place, Location, Condition/mood, Emotion.
¡Vamos! (ir) = "Let's go!" — used constantly in everyday speech.

All these verbs are completely regular in every form except yo, which changes its ending to -go.

Compound verbs follow the same rule — only the yo form changes:
tener → contengo, mantengo, obtengo, retengo…
poner → compongo, dispongo, expongo, propongo…
venir → convengo, intervengo, prevengo…
hacer → deshago, rehago, satisfago…
decir → predigo, contradigo…

Verbs ending in a vowel + -cer or -cir insert z before the c in the yo form only — all other forms are completely regular.
Pattern: conocer → yo conozco  ·  conducir → yo conduzco

Compounds follow the same rule — only yo changes:
conocer → reconozco, desconozco…  ·  aparecer → desaparezco…  ·  producir → reproduzco…
⚠️ Watch out: vencer and convencer end in consonant + cer (ncer) — the c simply becomes z, no insertion: venzo, convenzo.
⚠️ cocer (to cook) looks like it belongs here but it is a boot verb (o→ue) with a plain -zo ending — yo cuezo, not cuezco. The -zco pattern only applies to verbs derived from Latin -scere (conocer, merecer, etc.); cocer is not one of them.

The stem vowel changes in 4 forms (yo / tú / él / ellos) but stays unchanged in nosotros and vosotros — draw a “boot” around the four changed forms to see the pattern.

✶ Dual-behaviour -ir verbs — boot pattern in the present, but one step further in the preterite & gerund: sintió / sintiendo (not sentió), durmió / durmiendo (not dormió).

† Impersonal — 3rd person only: nieva (it snows), hiela (it freezes), llueve (it rains).
‡ Oler adds h- before the diphthong: huelo, hueles, huele — olemos, oléis, huelen.