The poem opens on a beach at night. The exact location is unidentified, just as the traveller and the town will later go unnamed. These generic details make the poem's scenario seem timeless, universal, and potentially symbolic.
Line 1 repeats the title word for word, establishing a refrain that will echo at the end of each stanza. This repeated line, with its strong rhythm and description of a repetitive process (that is, rising and falling), provides a steady "backdrop" against which the poem's changes take place.
Rhythmically, the line itself seems to "rise" toward and "fall" away from the grammatical pause (caesura) in the middle of the line. This pause is punctuated by a comma that links two independent clauses (clauses that could stand alone as complete sentences). A semicolon would be the more standard punctuation to use here, but the comma creates a softer pause, a smoother shift, that seems to fit the way the tide flows smoothly back and forth.
Line 1 establishes a strong meter of four beats per line, which continues throughout the poem:
The tide rises, the tide falls,
The twilight darkens, the curlew calls;
The poem's meter is accentual but not quite syllabic. In other words, the number of stressed (accentuated) syllables remains constant from line to line, but the total number of syllables and the position of stressed syllables do not. This first line contains seven syllables, while the rest contain either eight or nine. Every line, however, contains four strong stresses. (Parts of the poem, such as lines 3 -4, follow the accentual-syllabic meter called iambic tetrameter.)
The language of line 1 is plain, like the language of the poem as a whole, but it immediately suggests symbolic possibilities. The ebb and flow of ocean tides is a conventional symbol of change in general, including changes of human life or human fortune (as in an expression like "a rising tide lifts all boats"). It's also a classic example of a cyclical process in nature. Between the title and the identical first line, the reader may already sense that this poem will deal with change or cycles of change.