▲我与黎老师的看法稍有不同。我觉得下面两句都是对，但意思稍有不同：
Should I be free tonight, I will come. (=If I should free tonight, I will come.) 我万一有空，我就来。（should为情态动词，表示“万一”，说明可能性很小，这是一种比较圆通的说法）
Should I be free tonight, I would come. (=If I should free tonight, I would come.) 我要是有空，我就会来。（典型的虚拟语气，表示与事实相反，暗示的事实是：我没空，来不了）
▲【参考】以下内容摘自《张道真英语语法》第141页：
从句中可能用should构成谓语，这时主句可以用虚拟语气，也可用陈述语气：
If it should be necessary, I could come at six. 如果确有需要，我可以六点钟来。
If we should fail in this, we are (would be) ruined. 如果这件事失败，我们就毁了。
If a serious crisis should arise, the public would have to be informed of its full implications. 万一发生严重的危急情况，应让公众了解它的全部涵义。
If she should be interested, I’ll phone her. 如果她竟然有兴趣，我可以给她打电话。
Let me know if you should hear some more news. 万一你听到更多消息你要通知我。
▲if条件从句含有should，而主句用陈述语气或祈使句的更多例句：
If I should see him. I’ll ask him to ring you. 如果我能见到他，我将请他给你打电话。
If you should be fired, your health and pension benefits will not be automatically cut off. 如果你被解雇，你的医疗和养老保险金不会自动中断。
Should you lose your ticket call collect on STA’s helpline. 若遗失机票，请拨打STA的免费求助热线。
Should you buy a home from Lovell, the company promises to buy it back at the same price after three years. 如果你从洛弗尔公司买房子，该公司承诺3年后以同样的价格再购回。