Contractions such as ettei, from että ei (that not), are written like compound words, but they are really expressions of their own type. They are pronounced like simple words, e.g. ettei has no secondary stress, and the phonetic syllables are et and tei. In word division, both et-tei and ett-ei are accepted, though such words are best left undivided.
The difference between legato pronunciation and a contraction is largely a matter of definition: do we treat two words as separate, just pronounced together, or as fused together to form a single word?
In contractions, the second part does not usually adapt to vowel harmony. We normally say jotteivät (= jotta eivät), though jotteivat is used, too.
In standard language, the following words form contractions with the indicative form of the negation verb (en : et : ei : emme : ette : eivät): ehkä, että, jotta, koska, miksi, mutta, siksi, vaikka. In the contraction, the final vowel of the first word is lost, e.g. ehkei, ettemme.
For että and jotta, contractions are mostly automatic, i.e. take place whenever the word is followed by the negation verb. However, a contraction is sometimes avoided for stylistic reasons or for emphasis; e.g. että emme is possible instead of ettemme, just not common. For the other words, contractions are less automatic, but still used more often than not.
Contraction is common, partly even automatic, also in cases where a short word would appear between the contracting words. For example, it is possible to say En tiennyt, että hän ei tule, but is more normal to contract että and ei even “over” the word hän, yielding En tiennyt, ettei hän tule. Such contractions are more common in written language than in speech.
The words ellei and jollei, both meaning “if not” or “unless”, and their inflected forms like ellen are historically contractions and act as contractions. The first parts do not appear in modern language as separate words. (The word jolla exists, but in a different meaning, not as meaning “if”.)
These contractions are normally automatic: if a sentence would contain jos ei (or jos en etc.), either ellei or jollei (or ellen or jollen etc.) is used instead.
There is no substantial difference between ellei and jollei. The former is much more common. The latter might be seen as somewhat more emphatic.
In spoken language, many other contractions appear, too, such as täällon = täällä on. In speech, the words jos and kun often form contractions of a different kind with the negation verb: instead of omission of a vowel, the final consonant is doubled. Examples: jossen = jos en, kunnet = kun et.
Thus, the conjunction jos (if) and the negation verb can be combined with each other in three ways: ellei, jollei, and jossei, where the last one is colloquial only. The uncontracted jos ei is possible, too, but normally not used.