弾く as ひく or はじく: playing, repelling, and flicking
Distinguish the two readings of 弾く through nightly piano playing, a first public guitar performance, a rain-repelling coat, and a flicked marble. Preserve habits, properties, completed actions, and the scope of 'first'.
ひく: playing a musical instrument
弾く read as ひく refers to playing an instrument in these examples. 「姉は毎晩ピアノを弾いています。」 means “My sister plays the piano every night.” The sister plays piano every night. 毎晩 gives a repeated nightly habit, rather than one completed performance in the past.
「人前でギターを弾くのは初めてです。」 means “This is my first time playing guitar in front of people.” The first-time condition applies to playing guitar in front of people. It does not establish a first-ever attempt to play guitar, and it should not be retold as a public performance that has already ended.
はじく: repelling water or flicking a marble
The same written form 弾く also has the reading はじく. 「新しい上着は雨水をよく弾く。」 means “The new coat repels rainwater well.” The new coat's ability to repel rainwater is described as a current property. Its object and meaning differ from playing piano with ひく.
「指でビー玉を弾いて円の外へ出した。」 means “I flicked the marble with my finger and sent it out of the circle.” The speaker flicked the marble with a finger, sending it outside the circle. This is a completed action and its result, unlike the continuing property described for the coat.
Use reading, object, and time together
The piano and guitar examples use ひく; the rainwater and marble examples use はじく. The shared spelling does not make the two readings or meanings interchangeable. Nightly piano playing is a repeated action, the coat repelling water is a property, and sending the marble out is a past action.
Check the scope of 'first' as well. The guitar example concerns a first time playing in front of people, without denying all earlier guitar playing in other settings. Preserve that public-performance condition alongside the different readings, objects, habits, properties, and action results.
How is 弾く read in 「姉は毎晩ピアノを弾いています。」 and 「新しい上着は雨水をよく弾く。」?
Show the answer and why
It is ひく for playing the piano every night and はじく for the new coat repelling rainwater well. The spelling is the same, but the readings and objects differ. 毎晩 in the first sentence means every night.