不定詞片語置於句首表示目的:

例:

If you want to prepare a meal for about 50 guests, you should start by calculating how much food you want to prepare per guest.

=To prepare a meal for about 50 guests, start by calculating how much food you want to prepare per guest.

例:

To succeed,you should work hard.

=If you want to succeed, you should work hard.