A Greek translation of Hebrew almah is parthenos, which can mean both "young woman" as well as "virgin." There's a bit more detail in this thread . More on reddit.com

That's absolutely correct: Isaiah 7 is not about Jesus. It's a prophecy that was given to Ahaz. It even goes on to say "before the boy knows enough to reject the wrong and choose the right, the land of the two kings you dread will be laid waste". So it pretty clearly isn't about Jesus, since it was fulfilled quite quickly. So what's going on in Matthew then? Notice that not even Matthew calls it a prophecy. Usually by "prophecy" we think of "an event predicted before it happens and then it happens". But Matthew does say it's "fulfilled". So Matthew sees the birth of Jesus as "fulfilling" the passage. This is a very interesting word, because it's also the word that Matthew uses for Jesus coming out of Egypt, by saying that Jesus fulfilled Hosea 11:1. This is crucial. Hosea 11:1 most certainly isn't a prophecy, as it's about a past event: the exodus of the Hebrews out of Egypt. So it's a sentence that talks about a past event, and yet Matthew still says that Jesus fulfils it hundreds of years later. I think what's happening here is something more like Matthew saying "The story of the scriptures find their ultimate completion in Jesus". Ahaz had a child born that would signify that they would be saved from their enemies. So too Jesus being born would bring salvation. Matthew also quotes Jesus as saying that He came to fulfil the Law of Moses, which should probably inform how we read these passages both ways. It's also worth nothing that while παρθένος is explicitly a virgin, almah would have most probably denoted a virgin. It doesn't mean virgin by definition, but culturally, it would have. Perhaps kind of like how "homeless" doesn't mean you don't own an investment property that you're currently renting out, but it strongly implies it. More on reddit.com

How do you know if it was meant to be taken literally or not?

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