Elsewhere, Robert says, "kalleh mentions "risus sardonicus." I had previously only known the form "rictus sardonicus," which seems to be associated with death. Is there a grammatical distinction in Latin, which makes "rictus" the gone-forever form?!"

And jheem answers, "No, they're both past participles of two different verbs: ringor (rictum) 'to gape, open wide the mouth; to snarl' and rideo (risum) 'to laugh'."

Would that "rideo", with its D, be the root of 'ridiculous'?

Secondly, how did ridio change to risum, and by the same token, how did ringor change to rictum? Or is it just that you are just giving, in each case, the past participle and the infinitive forms?

how did ridio change to risum, and by the same token, how did ringor change to rictum? Or is it just that you are just giving, in each case, the past participle and the infinitive forms?

Rideo and ringor are both the 1st person present indicative forms of the two verbs. The tradition when giving Latin verbs (e.g., in a dictionary) is to give four forms: 1st person singular present indicative, infinitive, 1st person singular present perfect indicative, and neuter singular past participle. The gloss (English meaning) is usually given as the infinitve.

The forms I cited are the facts for Latin: i.e., rideo ~ risum (literally, 'I laugh' ~ 'laughed') and ringor ~ rictum (lit., 'I gaped' and 'gaped'). They're irregular verbs in Latin, but the reason historically for the forms is: rideo is what in Indo-European historical comparative linguistics is called a sigmatic aorist. The term comes from Greek but the phenomenon shows up in other IE languages. Sigmatic because an s (sigma in Gk) in the aorist (a tense; Gk like some other languages differentiates between a perfect and an aorist tense, both of which kind of map to English's preterite (past) tense. It's actually much more complicated because IE verbal tenses show traces of tense (past, present and future) as well as aspect (perfective and imperfective).

OK, why do the endings for rideo and ringor which are both 1st person singular present indicatives look different? Because there's a group of Latin verbs called deponent verbs which are active in meaning but passive in form. There is another important verbal category called voice (active, middle, and passive). Active verbs usually end in -o for the 1st person singular present indicative active (e.g., amo) and -or for 1st person singular indicative passive (e.g., amor 'I am loved').

So, why are the verbs irregular (leaving aside the question of deponent verbs)? Well the neuter past participle for rideo was probably *ridsom at some point (the asterisk is a convention to indicate the the word doesn't exist in the corpus of Latin, but is reconstructed along historical comparative lines. The -d- was assimulated. For ringor the form was probably *ringtom, but the -n- went away possibly due to stress and the -g- devoiced to a -c- because of the -t-. The suffix for the past participle is *-to- which shows up in loads of other IE languages.

I'm doing this from memory and may have got bits wrong (esp. the -n- bit), but I'll look them up in the reference books tomorrow and correct any problems. Hope that helps.

Probably the -n- is a present tense infix. Numerous Latin verbs have this, such as tango - tetigi - tactum, the stem being tag- 'touch' (but cognate with English 'thatch', not 'touch'). Some Greek verbs also had the present infix: lambano - elabon 'take'. I suspect stand - stood is the only example in native English, but as that's on its own I won't swear to it, as I'm doing this from memory too.

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Would that "rideo", with its D, be the root of 'ridiculous'?

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That's right. It is from the Latin ridiculus "laughable", from ridere "to laugh".

Probably the -n- is a present tense infix.

That's it; the nasal infix. Not too important in Latin, and in fact it's been regularized in some paradigms to the non-present tenses, e.g., jungo 'I yoke' ~ junxi 'I yoked', but jugum 'the/a yoke';

There's three different classes (native grammarians' term for conjugations) in Sanskrit that have the nasal infix in the present: classes 5, 7, and 9: s'ru root: 'to hear', s'r.n.oti 'he hears' ~ s'us'rAva 'he (has) heard'; ric root 'to leave', rinakti 'he leaves' ~ rireca 'he (has) left', but cf. Greek leipo I leave' (as in lipogram); pU root 'to cleanse', punAti 'he cleanses' ~ apipavat aorist 'he cleansed'. In some verbs, the infix was just an -n-, and in others it seems to have been an -ne-.