A vav-conversive (also called a “vav-consecutive”) is a conjunction usually translated “and”. It is easily distinguished from a simple vav conjunction since it always has a vav with a patah under it and a dagesh in the following letter.
וְשָׁמָֽיִם a simple vav conjunction
וַיִּיצֶר a vav-conversive
The key point to remember is that a vav-conversive converts a prefix form from future tense to past tense (these forms are often called a “preterite”).
The following rules are also helpful:
- It is only ever added to the prefix form form of the verb.
- The dagesh from the vav-conversive will never appear in the first letter of the verb.
- A vav-conversive will shorten a verb every chance it gets (KHW 32.3a). See the third-ה paradigm for an example (KHW p448).
More detail on the syntax of these forms is here.
See also vav-reversive.