We don't have any helping verbs attested in Sindarin that we know of. But, we extrapolate a few that should exist outside of our current knowledge of Sindarin. They are *law "no/not/don't" and *ce "may/might." Our ideas on where these would go is based on Quenya.
The trick to figuring out where these go is figuring out what the predicate is, because these two helping verbs go directly before the predicate. The verb, its direct object, indirect object, prepositional phrases, adverbs, and all of the affixes that modify the verb are what make up the predicate. The subject is not included in the predicate, unless it is a pronoun-suffix on a verb.
Lenition follows the helping verbs, but only if the following word isn't already lenited. You can't double-mutate a word.
*Law gin enengin. - I didn't kill you.
*Ce gi ndegin. - I might kill you.
Îg *ce nastar i thail lîn. - Thorns may stab your feet.
You can also use these words with the copula To use them, put them between the two words connected by the copula, and mutate the word following *law/*ce. The adjectives would still become plural to match the noun that they are describing.
I ethir *ce geredir. - The spy might [be] a craftsman.
Ci *law vain. - You [are] not beautiful.
Iñ gwind *ce dynd. - The maidens may [be] tall.
If you have both *ce and *law on the same verb, put *law before the verb and *ce after it.
*Law hen anírol *ce sîr. - You might not want this today.