The word “virgin” in Isaiah 7:14 is almah, which is not actually the Hebrew word word for “virgin,” which is bethulah. Does this mean that Isaiah 7:14 is not a legitimate prophecy? Not at all.
Therefore the Lord himself will give you a sign. Behold, the virgin shall conceive and bear a son, and shall call his name Immanuel.Isaiah 7:14
Some argue that since the word translated as “virgin” actually means “young woman,” this is actually not a legitimate prophecy.
The word translated “virgin” in Isaiah 7:14, ”almah”, actually means “young woman of marriageable age.” Although this does not necessarily mean that the woman is a virgin, it certainly does not exclude that possibility.
The Septuagint, a translation of the Hebrew Bible into Greek in about 200 BC, translates almah as parthenos, which is the Greek word for virgin. The Septuagint translation is what Matthew is using in Matthew 1:23, when he quotes the prophecy.
The probable reason that Isaiah used the word almah instead of bethulah was to emphasis the fact that the virgin would also be a young woman. The word in Isaiah 7:14 can certainly include the meaning of “virgin.”
There are some foundational principles that apply to all alleged and apparent contradictions in the Bible. To read more, see Bible Difficulties: Foundational Principles.
To read more answers to alleged and apparent contradictions in the Bible, see “Contradictions” in the Bible Answered.
These books are also excellent resources: