*Eres* most likely<sup>1</sup> comes from the 2nd person singular future active indicative of sum, *eris* <sub>("you will be")</sub>.

*Sois* is comes from the Late Latin form \**sutis* (formed by analogy to *sumus*)<sup>2</sup>:

> ESTIS (re-placed by *SUTIS, analogical to SUMUS) > OSp. *sodes* > ModSp. *sois*

---

<sup>1. [*On the Origin of Spanish eres*](https://www.jstor.org/stable/475003), Griffin (1994)</sup>  
<sup>2. [*The Origin of Spanish "ser": A Phonosyntactic Analysis*](https://search.proquest.com/openview/2c6c45288813fd5bd7e9e1098c878fa3/1), Joel Rini (1997)</sup>

<sup>Note that [some other tenses of *ser*](https://linguistics.stackexchange.com/questions/27254/are-the-english-words-essence-and-essential-related-to-the-spanish-word-ser/27262#27262) (future indicative, present subjunctive, conditional, imperative, infinitive, past participle, gerund) aren't related to *sum* at all, and are inherited from the suppletion of multiple different verbs in Latin (*ire, vadere, sedere*).</sup>