Complex words are words composed of two or more grammatical units. So, for example, the word "speech" is a simple word, because it is composed of a single grammatical unit, but the words "speechless" and "speechlessness" are complex words being composed of two and respectively three grammatical units each.
It is difficult to decide whether a word should be considered simple or complex because the vast majority of the polysyllabic words of English language have come from other languages. The way these words are constructed is easily recognizable. For instance, the words "commit", "submit", and "permit", which are borrowed from Latin, are formed by adding the prefixes "com-", "sub-", and "per-" to the stem word "mit". Examples can continue with words that have come from other languages but the point is that we can not treat each separate grammatical unit of each language as a separate grammatical unit of English language. So, drawing a clear line between simple and complex words is quite difficult and not always useful.
Affix words are words made from a basic stem word with the addition of an affix. Affixes are of two types in English: suffixes and prefixes. A suffix comes after the stem word (example: speechless) while a prefix comes before the stem word (example: permit).
Compound words are those words that are formed of two or even more separate independent English words. For example, "ice-cream" and "armchair" are compound words.
Affixes have one of three possible effects of word stress.
The prefixes have no comparative regularity or predictability of suffixes. Therefore, the best way of treating these words is to say that stress in words with prefixes is governed by the same rules as the rules for words without prefixes. So, there is nothing to say more about stress within words with prefixes. However, things are different in words with suffixes and compound words.