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"), with sedere, "to sit," also contributing to the historical development of ser. This is not a present-versus-everything-else split: the RAE describes roots such as er- in era and fu- in fui and fuera as well.
The contrast between soy and sería reflects different historical bases within one modern paradigm. That does not mean they remained separate verbs through most of Spanish history. Modern speakers learn them as forms of ser. That fusion is a separate question from what ser actually does in a sentence, which the ser vs. estar comparison covers in full.
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...) reports a completed state in the past: Fue un buen año. (It was a good year.) These forms are notably identical to the preterite of ir ("to go"), so 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.)
Imperfect subjunctive has two complete sets of endings, fuera, fueras, fuera... and fuese, fueses, fuese..., and they are interchangeable: Si yo fuera rico and Si yo fuese rico both mean "If I were rich." The -ra set is far more common in speech on both sides of the Atlantic; the -se set survives in writing and in Spain. RAE prints them as a single tense with two forms per slot, which is why the table above gives them on one row.
Future subjunctive (fuere, fueres, fuere...) is the one paradigm here you will almost certainly never need to produce. It has dropped out of the spoken language everywhere, and survives mainly in legal and administrative Spanish and in a handful of fixed expressions such as sea lo que fuere ("be that as it may"). It is in the table because RAE still prints it, and because reading a contract or an old text will eventually put it in front of you.
Affirmative imperative has no yo form, because commands aren't given to yourself. ¡Sé fuerte! (Be strong!) uses the irregular tú command form sé, spelled identically to sé meaning "I know" (from saber). Both verb forms need the written accent; context distinguishes the command from the statement of knowledge.
Negative commands are not built from the imperative at all. Spanish forms them with no plus the present subjunctive, which is why the negative row reads no seas, no sea, no seamos, no seáis, no sean rather than a negated sé. This is a general rule, not something special about ser.
The compound tenses: haber plus one unchanging word
Eight of the nineteen blocks above are compound tenses, and they are the easy eight. Every Spanish compound tense is a conjugated form of haber followed by a past participle, and a participle sitting after haber never agrees with anything, so ser contributes exactly one unchanging word to all eight of them: sido. Learn haber once and you get he sido, había sido, hube sido, habré sido, habría sido, haya sido, hubiera sido and hubiere sido for free.
Three useful starting examples are these. He sido (present perfect) reports a state that still touches the present: He sido profesor durante veinte años. (I've been a teacher for twenty years.) Había sido (pluperfect) sets one past event behind another: Ya había sido director cuando lo conocí. (He had already been director when I met him.) Hubiera sido (pluperfect subjunctive) carries counterfactuals: Hubiera sido mejor callarse. (It would have been better to keep quiet.)
Other compounds include everyday forms such as habré sido, habría sido and haya sido. Two forms are much more restricted. The preterite anterior hube sido is essentially literary and appears after time conjunctions like apenas or después de que; modern Spanish uses the plain preterite or pluperfect instead. The future perfect subjunctive hubiere sido belongs to the same legal register as fuere.
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 has an irregular voseo present form. For regular -ar and -er verbs in the common stress-final voseo pattern, replacing infinitive -r with -s gives forms such as hablás and comés; regular -ir verbs take -ís, as in vivís, 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.
This is not a dialect curiosity that reference works leave out. RAE's own conjugation table puts sos on the same line as eres, labelled "tú / vos," rather than treating it as a footnote. The affirmative command also matches tú: the voseo imperative of ser is sé, identical to the tú form, which is unusual, since most verbs take a distinct stressed voseo command.