The speaker begins the poem with a description of the soldiers' mental state and physical environment. Sitting in frigid trenches during World War I, the soldiers are mentally and physically exhausted. Their "brains ache" from the constant vigilance required of them in case of attack, and they must stay awake in the cold, even though "the night is silent." In fact, it is that silence itself that is so worrying, because the soldiers have no way of knowing where the next attack will come from, or when it will happen. Attempts to light up the battleground with "Low drooping flares" only serve to make the landscape more confusing.
What's more, nature itself seems to be working against the soldiers. The wind is personified as being without mercy, metaphorically stabbing the soldiers with its chill. This is the first indication that nature might be just as dangerous to the soldiers as any enemy army. Indeed, the "exposure" of the poem's title refers, at least in part, to the risk the soldiers face by being literally stuck out in the cold.
These lines also capture the strange duality of emotions the soldiers feel in this situation. They are both "curious" and "nervous"—likely curious because they've been doing nothing but wait in the freezing night air for hours, and nervous because, if something does eventually happen, it's probably going to involve immediate danger. The alliteration of the /w/ sound across the stanza further reflects the exhausting tension that the soldiers feel: they are at once "Wearied," "awake," "Worried," and "whisper[ing]."
Also note the hissing sibilance in lines 1 and 4: "merciless iced east winds that knive us" and "silence, sentries whisper, curious, nervous." The proliferation of /s/ sounds lends these lines a hushed and menacing quality that adds to the stanza's sense of weary anticipation.
Despite all this tension and vigilance however, "nothing happens." The speaker closes the stanza with the first instance of a refrain that he will repeat throughout the poem. The implication of this refrain extends beyond this specific wartime scene: by the end of the poem, it'll be clear that this phrase—"But nothing happens"—could just as well refer to the entirety of war itself. In other words, war is futile and meaningless; all this suffering changes nothing.
Also present in these lines is Owen's characteristic use of slant rhyme, seen between the word pairs "knive us"/"nervous" and "silent"/"salient." It's also as if the speaker is simply too weary to make his rhymes full and perfect. The poem almost rhymes but doesn't quite get there, adding to its sense of disorientation and unease.