Rashi lays down a rule of grammer commenting on Brashith 29:15. "And Lavan said to Jacob 'Just because you are my brother should you work (Hebrew: Vavad'tani) for me for nothing? Tell me what your wages are?'
Should you work for me (Vavad'tani) : RASHI: This is the same as ("V'ta'avdaini") 'And you will work for me.' So to in the case of every verb in the past tense, if one adds the letter "vav" at the beginning of the word, it changes the word to the future tense. "WHATS BOTHERING RASHI"
The vav conversive can function as an "and" depending on context of the passuk and where the cantillation mark is placed which places emphasis on and accents the particular word. For instance, Parshah Shmoth 3.15 Le'Olam has the accent on the latter syllable while the vav is totally missing from that word (Le'Olam) and is transposed to the very next word in the passuk: ve'Zeh "and this" which is superfluous because the cantillation for LeOlam takes the disjunctive Zakef katan. This clairifies our understanding of the latter half of the passuk, (3.15) "and this is My Memorial (how I should be remembered and mentioned) in every generation." remember and mention- in the merit of the patriarchs, we mention and remember them when we discuss Torah: "The G-D of Avraham, the G-D of Yitzqach and the G-D of Ya'acov."
This is a poor example of the use of the vav conversive changing the past tense into the future tense "and this" of 3.15 while also functioning as the conjunctive.
If we were to read from the very end of the passuk backwards to the word LeOlam, in 3.15 we can better grasp how the vav functions as both the conjunctive and as the inversive or conversive to change LeOlam past tense into future tense of zichri in the latter half of the passuk.
This is an instance where the vav functions to change present tense LeOlam into future "I will be remembered", zichri of 3.15 while simultaneously functioning as the conjunctive "and" (of and this in 3.15).
As I understand the passuk, 3.15 is telling us to remember the merit of the Patriarchs, each time we think of or mention "HaShem," that HaShem is unchanging LeOlam! His Name is eternal kindness Tehillim 136. That it was a eternal kindness, a blessing to reveal Himself to the Patriarchs and that unchanging aspect of the meaning of HaShem is associated with the Altar or place of sacrifices. Brashith 8.20, 21 Shmoth 20.20 Here, the passuk is telling us about an aspect of the unchanging nature of the Holy One! Which is that the G-D of Avraham, Yitzqach and Ya'acov is a G-D of Blessing.
The vav simply acts as a siman, as it were, for the missing vav of the word LeOlam. It functions as both a conversive vav and as a conjunctive "and" at the same time even though it is an unnecessary siman given that LeOlam takes a disjunctive Zakef katan cantillation mark.
This is really a poor treatment of the subject matters (above) on my part. The best advice I can give is read the article linked above What's Bothering Rashi.