P26 · The three most common words are, of all things, the least well-behaved — road, day, eye

Line one: why the most common is the least regular.

"Have you noticed that English's most irregular words — be, go, child, foot — are all used constantly? Sanskrit is the same.
Road (path), day (ahan), eye (akṣi), plus the cow (go／โค) you learned in P16 and 'man (pums)' — these words are used so much, and are so old, that they never got smoothed out by later regularizing rules, and they've kept their vintage parts intact. The good news: precisely because they're common, you'll run into them again and again in reading — just recognize them, no need to recite the full paradigm today."

Line two: path's two bodies — chest-out and tucked-in (recycling P20's two postures).

"Path has two body frames (Ruppel Ch.40):
Chest-out पन्थान-: appears only in the nominative, vocative, and directional hat (the 'nom·voc·acc' group across singular/dual/plural) — पन्थाः／पन्थानौ／पन्थानः／पन्थानम्. Note the special nominative singular पन्थाः: it ends with only one breath-hat -āḥ, and carries no second nasal (it is not *पन्थान्स्).
Tucked-in पथ-: this thinner form is used for every other hat — instrumental पथा, locative पथि, genitive पथः.
One small move: when the tucked-in form meets a heavy consonant-initial hat (the -bhiḥ/-bhyām/-su group), it inserts an i first: पथिभिः pathibhiḥ, पथिषु pathiṣu."

Line three: the contraction-hiding tucked-in form — akṣi and ahan (another way of playing tucked-in).

"Eye (akṣi) and day (ahan) are neuter nouns, well-behaved most of the time — but before a vowel-initial hat, the middle sound gets contracted and replaced with a nasal-carrying tucked-in form:
akṣi → अक्ष्ण- (akṣṇ-): instrumental अक्ष्णा akṣṇā, genitive अक्ष्णः. In the same nest: दधि (curd) → दध्न-, अस्थि (bone) → अस्थ्न- (SKT 26.3's set of 'an/i doublets' — look them up under the i-form in a dictionary).
ahan → अह्न- (ahn-): instrumental अह्ना ahnā, locative अह्नि ahni (also अहनि).
Day's most famous trick: the subject-hat/directional-hat singular is not *ahan, but अहः ahaḥ — the tail -n swaps for a single breath-hat -ḥ. In compounds it also shows another face, अहर्/अहः (as in अहोरात्र ahorātra, 'day and night')."

path full paradigm (पथ्- "path," Ruppel Ch.40 entry, pending Heritage machine-checking)

(Chest-out only in the top two rows' 'nom·voc·acc group'; everything else is the tucked-in पथ-, with i inserted before a heavy hat.)

Three irregular words, three faces (this lesson's main table — recognition, not production)

"Scan the rows for the pattern: path shifts its body frame (chest-out/tucked-in), ahan／akṣi contract their middle sound (a nasal-carrying tucked-in form); before a heavy hat (instrumental plural -bhiḥ), all three words return to their plainest tucked-in shape. Just remember 'which word plays which game' — no need to memorize the full paradigm" (Ruppel explicitly says rote-memorizing the whole table for this class of words doesn't pay off).

▸ A point to lighten the load (Ruppel/Goldman consensus): passive recognition takes priority over active production. These words are common — you'll meet them again and again in reading; the goal is to see अक्ष्णा and recognize it as "eye · instrumental hat," not to recite it from memory with your eyes closed. The full table is laid out in Crystallization Lesson Seven, for reference whenever needed.

Building blocks (use-first, analyze-later — this lesson's set): प्रतिदिनम् pratidinam ("every day," prati + dina).

"ahan never made it into Thai, but its well-behaved synonym dina did — ทิน←dina (it's right there in ปฏิทิน, 'calendar'). प्रतिदिनम् = 'each and every day,' use-first outside the story. The old, irregular ahan was too unruly, so Thai went around it and borrowed the tidy dina instead — a living example of Line One's point."

TPRS wrap-up: "What did the boy walk out onto? Whom did he see on the road that day? What did the seer use to look at him? Which path did the seer say was the best?" — students answer with पन्थानम्／अह्नि／पथि／अक्ष्णा／पन्थाः, choral recitation of the four-sentence sandhi version.