Theory:

What would bechance at Lyonnesse
While I should sojourn there,
No prophet durst declare;
Nor did the wisest wizard guess
What would bechance at Lyonnesse
While I should sojourn there.
Explanation:
 
When one sets out on a journey, there are two emotions that follow - curiosity and excitement. Human beings cannot foresee or foretell the future. Life is full of unexpected twists and turns and that is what makes it more beautiful. If everyone knew what the outcome of an action was, then life would become boring. Above all, knowing the end results would only make life purposeless and meaningless. It is not the result but the journey that matters. When Hardy set out for Layonnesse, he is unsure as to what the journey holds in stock for him. He knows that his stay there is only temporary. But he is also curious to know what his stay in Layonesse might teach him.
 
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The journey is what matters
 
Hardy says that no prophet can declare what would happen in Layonnesse, as it is a mystical city. Prophets are people who are considered to be closer to God. It is believed that they can see what happens in the future. But Layonnesse is a place beyond the reach of any prophet's vision. The prophet here can also be a reference to the priest in the church that he is about to visit. Great things can only be felt and not seen or foretold. He also gives another example of how a wizard, in spite of his magical powers, cannot tell him what he might encounter in Layonnesse. A wizard is supposedly capable of performing magic and therefore is considered to possess powers to foresee the future. But the magical and mystical element in Layonnesse is far superior to any wizard's magic spells. Therefore the second stanza shows that the poet is curious and excited about his journey. He is hoping to eliminate all his lonesomeness with the help of the journey.
 
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Wizard
 
Meaning of difficult words:
 
S.No
Words
Meaning
1
BechanceHappen or find
2
SojournA temporary stay
3
ProphetA follower of God who spreads God's words
4
DurstDare to do something
5
DeclareSay or announce something with confidence
6
WizardA person who has magical powers
7
ForeseePredict the future
Reference:
National Council of Educational Research and Training (2008). Honeydew. When I set out for Layonnesse - Thomas Hardy (pp. 105 - 106). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.