The active participle would probably take the ending -el

This part is rather hypotycal, there is not enough exemple to be 100% sure that this is exactly how it will react.

      dar- (halt)---- > darel (halting)
      tol- (come) ---> tolel (coming)

When the stem vowel is i, the ending will seems to expand to -iel

(There is probability that this will occurs only if the ending follow the consonants n, l, and r, and that the extra i materializing came form a older stage of the language):

      fir- (die, fade)--- > firiel (dying, fading)
      glir- (sing) also (recite poem) --- > gliriel (singing or, reciting)

(we have to be careful since the lengthened vowel will change the tense like in : tiriel "watching" from tíriel "having watched". palan-diriel means "gazing far away", palan-díriel has a perfective meaning: "having gazed far away".)

***This lengthening of vowels had probably occurred really early in the sindarin language that the subsequent changes affecting long vowels must also be taken into consideration. Earlier é, á, ó would be expected to manifest as í, ó, ú, :

      mad- (eat)---- > módiel 'for mádiel' (having eaten)
      nor- (run)---- > núriel 'for nóriel' (having ran)

***It seems that neither of the active participles so derived (in -el and -iel) have distinct plural forms.

The passive participle (or past participle) will be constructed by adding -en to the 3rd person singular past tense form (see rules above):

      dar- (stop)---- > 3 pers. sg. pa.t. darn 'he, she, it' (stopped) ---> passive participle darnen (stopped, halted)
      tir- (watch, guard)---- > 3 pers. sg. pa.t. tirn 'he, she, it' (watched, guarded)---- > passive participle tirnen (watched, guarded)

Again, when another vowel comes to follow them, final -nt, -nc, -mp, -nd, -m become -nn-, -ng-, -mm-, -nn-, -mm-, respectively:

      ped- (speak)--- > 3 pers. sg. pa.t. pent "he, she, it ''(spoke)--- > passive participle pennen (spoken)
      hab- (clothe)---- > 3 pers. sg. pa.t. hamp "he, she, it'' (clothed) ---- > pass. part. hammen (clothed)
      lav- (lick)--- > 3 pers. sg. pa.t. lam 'he, she, it' (licked)--- > pass. part. lammen (licked)

They will do their plural by -in, causing the normal umlauts: a and o both become e:

      dangen (slain)--- > pl. Dengin
      pennen (spoken)--- > pl. Pennin

The gerund, the verbal noun, that may also be used to translate English infinitives (see above). The gerunds of basic verbs are easily formed with the ending -ed:

      cab- (jump) ---> cabed (jumping) as noun, = (a jump, a leap")
      glir- (sing)--- > glired (singing)

Exercices

The active participle

ped- (speak) --- > _________________
gir- (shudder) --- > _________________
tir- (watch) ---> _________________

The perfective active participle

fir- (fade, die) ----- > _______________
glir- (sing/recite) ------> _______________
ped- (speak) ----- > _______________

The passive participle

sol- (close) --- >(plurial) __________________
dag- (slay) ---- > __________________
hab- (clothe) ---- > (plurial) __________________
redh- (sow) ---- > __________________
lav- (lick) ----- > __________________

The gerund

cen- (see, look) ----> ________________
tol- (come) ---- > ________________