Lenited after an imperative verb, if I'm correct ("...edro hi ammen").
Sí is here, si is now, according to the sources. Whether they were interchangeable, I don't know.

According to my understanding of PE17, Tolkien says that in LotR-style Sindarin "si" means 'in this place, here' independently of its length (sí could be a more emphatic form, it is under iambic stress both times it occurs), and "hi" for older thî, hî is interpreted as 'now'. Judging by examples, both always appear unlenited in LotR, presumably for both aesthetic purposes and not to confuse them with each other. Tolkien explicitly compares S. si/hi distribution to Q. sinomë/si and asks not to confuse Q. si 'now' with S. 'here' (27). I believe, we should never get "si" lenited for we only confuse the meaning this way. +Tamas Ferencz In 'edro hi ammen' he writes "hi" - "now" is used, again unlenited.

+Александр Запрягаев
well spotted. I admit I have forgotten that he discussed these adverbs in PE17. Thanks for the correction!

According to PE/17:67, however, "sī = now or here, but in Quenya usually 'now'. ... In Sindarin it usually = here." (bold emphasis by me). So it's not that simple. :)

+Lőrinczi Gábor Possibly, the "here" and "now" meanings were quite interchangeable in Elven minds, or Quenya influence could lead to Noldorin mis-use?..

+Александр Запрягаев Yes, that's quite possible.

+Lőrinczi Gábor Off-topic question: why does Tolkien mark i in "si" on p. 27 with both long and short marks above each other? Does he imply both vowel lengths are possible?

There is another note by Tolkien (VT49:34 note #21) indicating that Sindarin sî = "here" and hî = "now". I can't find any examples of Sindarin sî used for "now" in my notes. I think the evidence indicates that hî is a distinct word rather than the lenited form of sî, though possibly the two originated in the same primitive form.

As for long versus short i, my guess it that it would depend on whether or not the word was stressed in the sentence.

+Александр Запрягаев Yep.

+Paul Strack In Lúthien's song (LB/354), si is usually translated as "now". Of course, it doesn't prove anything. :)

+Paul Strack I tend to agree with short vs. long: in Lay of Leithian, si is unstressed by metre, and thus gets short. +Lőrinczi Gábor is there any 'official' translation, e. g. by Christopher Tolkien? Indeed, Salo's translation gives "now", but it was done way before PE17, and some of the existing translations just skip the word at all…

+Александр Запрягаев As far as I know, it is unfortunately untranslated, so there is no official translation.

I read the poem, and I think it makes sense to translate si either "here" or "now", so it perhaps contains no clues at all.

+Hjalmar Holm I think the meaning of si in Lúthien's song is highly dependent on whether ir means "the" or "when".

Or if it means "alone, only", as I have understood that it's glossed in VT 50. But actually, I don't think the exact reading of si is very important for the understanding in this poem.