The only actions in the first stanza are those taken by the earth as Arnold looks out his window upon the beautiful moonlit view before him. The moon, the tide, the waves, all of these elements are the ones that are being described as doing anything. The only human action suggested is when Arnold calls to his loved one, asking them to "come to the window" and "listen" to the waves, and smell the sweet air. The rest of the descriptions relate to nature. The sea is "calm," the moon "lies fair," the light "gleams," the cliffs "stand," the waves "draw back, and fling" the pebbles. All of these things are just poetic descriptions of the moonlit evening on a beach.
On a deeper level, it is suggested at the very end of the first stanza that the waves are, as they lap at the shores and toss the pebbles about, bringing in with them an "eternal note of sadness." They are ushering in the concept of "human misery and woe" that Arnold refers to in stanza two. Arnold can't help but feel melancholy as he views the scene, and thinks that the waves sound sad, and that they represent all of human sadness, which covers the entire earth. So, that suggested action, that the waves bring in sadness, introduces the theme that he will dwell on for the rest of the poem. I hope that helps. Good luck!
In the opening lines of his poem, Dover Beach, Arnold presents a naturalistic description of the calm and full Dover sea, the image of the moon on the straits, the moonlit shore, the gleaming light on the French coast, and the vast glimmering cliffs of Dover. In this nightscape, the action lies with the moving waves coming on to the shore and going back.
In line 6, 'Come to the window......', the poet suggests a piece of action to his beloved/companion ( presumably his newly-married wife ). The poet asks her to come to the window of the observatory to look at the sea, to hear the sound of the tides, and to enjoy the sweet night air. He further asks her to listen to the waves, to hear 'the grating roar of pebbles'. The pebbles drawn from the beach by the waves, and flung back on to the shore produce a frictional noise, a noise having 'a tremulous cadence slow', a music that the poet urges his companion to take note of : 'the eternal note of sadness'.
The action suggested by Arnold is a contemplation of the destiny of human suffering, a contemplation to be had of the close and intent observation of the sea and the melancholy music that the waves produce in terms of the perpetual to and fro motion of the age-old sea.