antaŭtempa

A real compound, and it decodes cleanly. Cut from the right, as compounding tells you to. The head is -a, so this is an adjective. Tempo at rank 98 is time. Antaŭ at 51 is before. Something antaŭtempa is before-time: it happened ahead of the time it should have. That is exactly what "premature" means, and it is exactly what the Latin praematurus behind the English word means too - prae, before, plus maturus, ripe. Two languages, the same idea, built with the same two pieces.

The linking vowel, and why there isn't one.Antaŭ is a preposition, so it joins the next root directly with no -o- in between: antaŭtempa, not antaŭotempa. Compare vaporŝipo, where two nouns meet and the -o- is dropped for ease, and patrolando, where it is kept. The rule is in the compounding article, and prepositional first elements are the easiest case: they never take one. Note also that the diphthong aŭ is one syllable, so the word has four - an-taŭ- tem-pa - and the stress falls on tem.

How it is actually used. Mostly of births and of deaths: an antaŭtempa naskiĝo is a premature birth and an antaŭtempa morto is an untimely one. The adverb antaŭtempe is the form you will need more often, because English "prematurely" usually attaches to a verb: li eliris antaŭtempe. Where the sense is "too early" rather than "before the proper time", tro frue is plainer and better. And do not confuse it with antaŭa, which simply means previous or foregoing; la antaŭa ĉapitro is the last chapter, not a premature one.