This verse contains four independent sentences. They are: 1) udakaṃ hi nayanti nettikā (irrigators lead water). The subject is the noun nettikā (irrigators, nominative plural). The verb is nayanti (lead, 3rd person, plural, active, indicative, present tense). The object is the noun udakaṃ (water, accusative singular). The particle hi (indeed) stresses the object and serves mainly for metrical purposes. 2) usukārā namayanti tejanaṃ (arrow-makers bend arrows). The noun usukārā (arrow-makers) is the subject of this sentence. The verb is namayanti (bend, 3rd person, plural, active, causative, present tense). Object is the noun tejanaṃ (arrow-shaft, accusative singular). 3) dāruṃ namayanti tacchakā (carpenters bend wood). The subject is the noun tacchakā (carpenters, nominative plural). The verb is namayanti (bend, 3rd person, plural, active, causative, present tense). The noun dāruṃ (wood, accusative singular) is the object in this sentence. 4) attānaṃ damayanti paṇḍitā (wise ones master themselves). The noun paṇḍitā (wise ones, nominative plural) is the subject here. The verb is damayanti (master, 3rd person, plural, active, indicative, present tense). The object is the noun attānaṃ (oneself, accusative singular).
Venerable Sāriputta once had a very young novice. On the eighth day after becoming a novice he was with Sāriputta on an alms-round. He observed irrigators irrigating the fields, arrow-makers making their arrows and carpenters working with wood. He asked Venerable Sāriputta if those things, which have no mind, could be guided to wherever one wishes. Sāriputta replied that it is so. The young novice then thought, "If those things, which have no mind, could be guided to wherever one wishes, then why could not I master myself?" He then asked permission from Sāriputta, returned to the monastery and diligently practiced. Very soon he attained the third stage of awakenment and was very close to attaining full arahantship. Sāriputta then returned and was going towards the novice's hut. The Buddha saw this and he also saw that the novice was just about to attain the goal, so he met Sāriputta outside and prevented him from going to the novice's hut by asking him various questions. The novice indeed attained arahantship very soon and the Buddha explained that the reason, why he kept Sāriputta outside, was to enable the young novice to attain his goal without being interrupted.