"Tie the knot" is a phrase that means to get married. It is thought to come from the ancient practice of physically tying a knot as a symbol of a binding agreement or pledge.
Here are some example sentences using the phrase "tie the knot":
"They decided to tie the knot in a small ceremony on the beach." (他们决定在海滩上举行小型婚礼。)
"After dating for five years, they finally decided to tie the knot." (在交往五年之后，他们最终决定结婚。)
"Do you think they'll ever tie the knot?" (你认为他们会结婚吗？)