像 we have no choice but to wait for his reply.（我们只得等待他的回复）这里

此处but为介词,意为"除...之外"
当but前面有动词do或do的其他变化形式时,只加动词原形.如:I did nothing but wait there for two hours.
而but前面无动词do或其变化形式时,则要加不定式.如:I couldn't say anything but to stand still silently.