同义词替换：

– happen to = accidentally, by chance, coincidentally, fortuitously

句型转换：

1. 原句：I happen to have an extra ticket.

转换后：I accidentally have an extra ticket.

I have an extra ticket by chance.

I coincidentally have an extra ticket.

I fortuitously have an extra ticket.

2. 原句：She happened to be at the store when I called.

转换后：She accidentally was at the store when I called.

She was at the store by chance when I called.

She was at the store coincidentally when I called.

She was at the store fortuitously when I called.

3. 原句：It happens to be a beautiful day today.

转换后：It accidentally is a beautiful day today.

It is a beautiful day by chance today.

It is a beautiful day coincidentally today.

It is a beautiful day fortuitously today.

通过这些替换和转换，可以灵活地使用不同的词汇和句型来表达相同的意思。