The Latin verb sum is also cobbled together from descendants of the two Proto-Indo-European roots es– and bhu–, even though it doesn't quite look that way. In Latin, the stem for the whole present system is from es–, and the perfect system stem is from bhu–.
Table 12-1 The Verb sum, esse, fuī , futū rus (to be)
The stem for the imperfect and future tenses experienced what linguists call rhotacism. It often happens in Indo-European languages that when an s finds itself sitting between two vowels, that s changes to an r. You can see this in English with the plural of “is” being “are,” and the plural of “was” being “were.” That's why e s am became e r am.
There are a couple more linguistic sleight-of-hand maneuvers that you might find interesting. For the imperfect of sum, cover the er– and replace it with a b–. Look familar? The same thing will happen if you do this with the future. Now cover the fu– in the pluperfect and future perfect and compare them with the imperfect and future. Presto! (Well, except for the –i– in erint.)
The forms for the perfect system don't need any explanation. The endings are regular. It's only the stem that is strange.
The English “be” and the Latin fu– don't look like they have anything to do with each other until you consider the relationship the letters b or p and f have between the languages. It usually happens that where one has a b or a p, the other has an f (e.g., f rā ter [“brother”], p ater [“father”], f erō [“bear/bring”]).