Non-believers are quick to assert that almah means "young woman" here -- to discredit the prophecy that Mashiach was "born of a virgin." However it seems obvious that in biblical times a young unmarried Jewish woman was assumed to be a virgin. The sign of Mashiach's birth was a miracle -- a virgin birth -- a child without an earthly father. Young married women giving birth would hardly be a sign -- it would be commonplace.
In Song of Solomon 1:31 and 6:8, Rashi makes almah mean "virgin." That fits. Yet in Isaiah 7:14, Rashi tries to say almah means young woman without virginity being implied. That is quite contrary to how he interpreted the same word in Song of Solomon. Further Rashi acknowledged that other scholars of his day
did understand almah in Isaiah 7:14 to mean "virgin."2
Does almah mean virgin or not? Let's look elsewhere in scripture. Almah is first found in Genesis 24:43, "behold, I stand by the fountain of water; and let it come to pass, that the virgin/almah that cometh forth to draw, to whom I shall say: Give me, I pray thee, a little water from thy pitcher to drink;" . Rabbis agree Rebecca's virginity was understood here. In Exodus 2:8 "And Pharaoh's daughter said to her: 'Go.' And the maiden almah went and called the child's mother" (maiden meaning Miriam -- Moshe's sister -- still a child herself). Rabbis agree this one can mean virgin too.
Probably one of the strongest argument for almah meaning "virgin" is that when the 70 rabbis translated the Septuagint (around 2nd century BCE) they translated almah as parthenos ("virgin" in Greek). There were perfectly good Greek words for young woman, but note, the rabbis chose the word for "virgin. So if modern Judaism wants to debunk almah by saying it can't mean virgin, we see this is newer thinking (not even popularized yet in Rashi's day) and that previously almah was understood to mean "virgin" -- even in Isaiah 7:14.
1:David Stern's Jewish New Testament Commentary copyright 1992 (4th edition 1995) p 7.
Almah And Isaiah 7:14
Ellen Kavanaugh
"Behold, the virgin/almah shall conceive and bear a Son and His name shall be called Immanuel." Isaiah 7:14
2: Arnold G Fruchtenbaum's Jesus Was A Jew copyright 1981 (3rd printing 1995) p 52.