sum

Learn the present tense of "sum" as a block, because recognising these eight strings is most of what reading early Latin consists of: sum (I am), es (you are), est (he, she or it is), sumus (we are), estis (you all are), sunt (they are). The infinitive is "esse", to be.

It is wildly irregular, and the other stems have to be learned rather than derived. The imperfect runs eram, eras, erat, eramus, eratis, erant (I was, you were, and so on); the future is ero, eris, erit; the perfect is fui, fuisti, fuit, with the infinitive "fuisse". The subjunctive supplies "sit" and "sint" in the present and "esset" and "essent" in the imperfect, and those turn up constantly inside "ut" and "cum" clauses.

Two habits to build. First, "sum" takes a NOMINATIVE on both sides, not an accusative: "Romanus sum", not "Romanum". Second, it is frequently used to mean "there is" or "there are" with no separate word for "there" at all.