The theme of 'The Wild Swans at Coole' is the passage of time.
In the past, the poet could count all the Swans before they all flew away. At that time, he was much younger and passionate.
At present, 19 years have passed after he last saw the swans, he can count there is 'nine and fifty' swans, the number of swans has been decreasing, though they are still 'beautiful' and 'mysterious'. It seems that they are not affected by time. But the poet's 'heart is sore', he becomes less passionate and becomes old day after day.
Then the poet thinks whether the swans will all fly away in the future, and he hopes he can always remember the beautiful scene as lively as he is seeing now near the lake. This shows that the poet doesn't want to grow old, he longs for youth and passion.
The regular rhyme scheme, ABCBDD and iambic meter, 434353 in each stanza might indicate the swans were unwearied of the time changing, because after 19 years, their appearance were still the same in the poet's eyes.
However, the deviation of rhyme scheme in last stanza was clearly showing us that the life isn't as beautified as the swans are showing, because the poet felt he was becoming older and older and the deviation meant his life was going to the end, his life was no longer fruitful and was full of flaws, such as unhappiness.
The use of imagery, especially 'numbers' was significant in showing how passage of time has affected the feelings of the poet. Such as 'nine-and-fifty', 'nineteenth', 'first', these numbers were odd, indicating that the poet felt alone throughout the nineteen years and since the numbers were decreasing, 59 to 19 to 1, it showed the decaying...