I visited the place where we last met.
Nothing was changed, the gardens were well-tended,
The fountains sprayed their usual steady jet;
There was no sign that anything had ended
And nothing to instruct me to forget.
The thoughtless birds that shook out of the trees,
Singing an ecstasy I could not share,
Played cunning in my thoughts. Surely in these
The last line  personifies  ‘nothing’. This blank, non-existent entity can’t therefore ‘instruct’ the speaker.  It comes clear to the reader that this person is grieving, trying to put out of her mind the memories, but is unable to. The mood of sadness is subtly established.