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.

上段节选中，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. 该句中:

If...shoud lie..., this was...是虚拟语气吗？ 是对将来的虚拟假设，if从句用should lie, 表明可能性极小，

但是主句是否使用了would/should/could/might+do呢？

主句中的this was - was 该如何理解？ 也是虚拟吗？怎么理解呢？为什么不是were?

请老师分析主句的虚拟形式。