Why ser breaks every conjugation pattern
Regular Spanish verbs conjugate by attaching a predictable set of endings to a stem — that's the whole system behind verbs like hablar or vivir. Ser doesn't follow that system because, historically, it isn't really one verb: Spanish ser is suppletive, meaning its paradigm was assembled from two different Latin verbs rather than descending cleanly from one. According to the linguistic research site Spanish Linguist, the present-tense indicative forms (soy, eres, es, somos, sois, son) descend from the Latin verb esse ("to be"), while nearly everything else in ser's conjugation — the infinitive itself, the subjunctive, the future and conditional stems, the tú imperative (sé) — descends instead from Latin sedere, "to sit."
That's the actual reason soy and sería don't look like they belong to the same verb at first glance: for most of Spanish's history, they effectively didn't. Two separate Latin verbs merged into a single paradigm, and Spanish speakers today just learn the result as one (extremely irregular) verb.
How to actually use each tense
Present (soy, eres, es...) states identity, origin, and inherent characteristics right now: Soy de Chile. (I'm from Chile.)
Preterite (fui, fuiste, fue...) — notably identical in form to the preterite of ir ("to go") — reports a completed state in the past: Fue un buen año. (It was a good year.) Context is the only thing that tells fue apart from "he/she/it went."
Imperfect (era, eras, era...) describes an ongoing past state with no marked beginning or end: Era muy joven. (I was very young.) If the preterite-versus-imperfect distinction itself is still fuzzy, ser's two very different-looking past forms are a good pair to drill it on.
Future (seré, serás, será...) and conditional (sería, serías, sería...) work the way they do for any verb: future for what will be, conditional for what would be.
Present subjunctive (sea, seas, sea...) shows up after expressions of doubt, wish, or emotion: Espero que sea verdad. (I hope it's true.)
Affirmative imperative has no yo form (commands aren't given to yourself) and uses a five-slot pattern instead of six: ¡Sé fuerte! (Be strong!) uses the irregular tú command form sé, which — confusingly for learners — is spelled identically to sé meaning "I know" (from saber), though the accent-free spelling is correct for both and context always makes the meaning clear.
A regional variant the table above doesn't cover: voseo
The table on this page uses tú for the informal singular "you," which is standard across Spain and most of Latin America. But in Argentina, Uruguay, and parts of Central America, everyday speech uses vos instead of tú — a system called voseo — and ser is one of only two verbs (alongside ir) that breaks the regular voseo pattern. Most verbs form their voseo present by swapping a final "-r" for "-s" and shifting the stress to the last syllable, but ser's voseo present is simply vos sos, not a predictable variation on tú eres.
In Rioplatense Spanish — the dialect spoken around the Río de la Plata basin, covering Argentina and Uruguay — voseo isn't a casual regionalism confined to friends; it's the everyday standard, used in schools, advertising, and television. A learner who only knows tú eres will still be understood there, but vos sos is what they'll actually hear.