你的句子有错，不定式作主语，应用单数动词（与不定式短语中作宾语的名词无关），所以你的句子应该是：
To make new friends is difficult. 结交新朋友很困难。
又如下面的句子也都是用单数动词：
To make mistakes is easy. 犯错误是容易的。
To tease animals is cruel. 逗弄动物是残忍的。
To refuse invitations is not always easy. 拒绝邀请有时候不容易。
To obey the laws is everybody’s duty. 遵守法律是每个人的义务。
To ask such prices is sheer robbery. 要这样的价钱简直是明火执仗的抢劫。
To teach foreign languages is not so easy as you think. 教外国语不是如你所想的那样容易。
To know these rules is one thing; to apply them correctly is quite another.知道这些规则是一回事;正确地运用它们又是一回事。
顺便补充一句，有时可见到两个并列的不定式作主语，此时谓语可能用单数（视为同一概念）或复数（视为两个不同的概念或动作）。比较：
To work all day and to go out to parties every night is simply burning the candle at both ends. 整天工作，同时还每晚出去参加派对，简直是过度消耗精力。
To eat to live and to live to eat constitute two different attitudes toward life. 为生存而食和为食而活是两种不同的生活态度。
