Passive participles in Sindarin (and Noldorin) are reasonably well attested. For half-strong and derived verbs, its formation is straighforward: add -en to the past tense, with the final -nt become -nn- medially as usual. The clearest Sindarin example of this is the half-strong verb covad(a)- “(make) meet” with past tense covant, passive participle covannen “met” (PE17/16, 158).
In addition to tense, Sindarin verbs are also inflected with subject suffixes in the first and second person. The full set of suffixes (and their conceptual development) is discussed in the entry on subject suffixes. The treatment of 3rd person is somewhat unusual, however. The 3rd singular inflexion has no pronominal suffix, as in: agarfant beth “he spoke words” (PE17/126).
There is no future tense in Gnomish or Early Noldorin, perhaps because the Welsh present tense can also be used to describe the future: strictly speaking the Welsh “present tense” is really “non-past”, just like the English simple present. The first mention of a distinct future form is in Quendian & Common Eldarin Verbal Structure (EVS1) from the late 1940s:
Sindarin (and it’s conceptual precursors) have two major classes of verbs: basic verbs (based on an unadorned primitive verbal root) and derived verbs (produced with some kind of verbal suffix). In the literature, these two classes are sometimes called i-stem verbs and a-stem verbs after their present tense forms, which frequently have i or a respectively. For example, this terminology appears in David Salo’s book, Gateway to Sindarin (GS/111), though I am not certain he first coined these terms.