怎么樣理解主句的虛擬語氣形式？

Over the green squares of the fields and the low curve of a wood there rose in the distance a gray, melancholy hill, with a strange jagged summit, dim and vague in the distance, like some fantastic landscape in a dream. Baskerville sat for a long time his eyes fixed upon it, and I read upon his eager face how much it meant to him, this first sight of that strange spot where the men of his blood had held sway so long and left their mark so deep. There he sat, with his tweed suit and his American accent, in the corner of a prosaic railway-carriage, and yet as I looked at his dark and expressive face I felt more than ever how true a descendant he was of that long line of high-blooded, fiery, and masterful men. There were pride, valour, and strength in his thick brows, his sensitive nostrils, and his large hazel eyes. If on that forbidding moor a difficult and dangerous quest should lie before us, this was at least a comrade for whom one might venture to take a risk with the certainty that he would bravely share it.



上段節選中，最后If on that forbidding moor a difficult and dangerous quest should lie before us, this was at least a comrade for whom one might venture to take a risk with the certainty that he would bravely share it. 該句中:



If...shoud lie..., this was...是虛擬語氣嗎？ 是對以后的虛擬假設，if從句用should lie, 表明可能性極小，

但主句是不是用了would/should/could/might+do呢？

主句中的this was - was 該怎么樣理解？ 也是虛擬嗎？如何理解呢？為何不是were?



請老師剖析主句的虛擬形式。



If on that forbidding moor a difficult and dangerous quest should lie before us, this was at least a comrade for whom one might venture to take a risk with the certainty that he would bravely share it.

if從句用should do形式，主句可以用虛擬語氣、陳述語氣或祈使語氣。本句中主句謂語動詞用was, 即主句用了陳述語氣，說明if從句為真實條件句，即從句的條件有達成的可能。

真實條件句中，主句可以不需要情態動詞。比如：

If you make one more mistake, you are fired.

你的句子就是類似使用方法，只是說話人的立場不是目前，而是過去，所以主句用was表示過去以后的時間。

另外，虛擬語氣形式were只用于從句，不會用于主句的。