Few verses have been disputed more that this verse when it comes to Bible translation and theology. I’ll try to keep myself to linguistics.
One of the ways to study the range of meaning of any word or phrase is to look at how it is used in context. When the dictionaries say one thing and the usage of the word suggests something else, I am not certain what to trust the most.
The Hebrew word ‘almah occurs the following places in the Hebrew OT. I’ll quote the RSV translation and indicate how the LXX translated the word:
Gen 24:43-44: behold, I am standing by the spring of water; let the young woman who comes out to draw, to whom I shall say, “Pray give me a little water from your jar to drink,” and who will say to me, “Drink, and I will draw for your camels also,” let her be the woman whom the Lord has appointed for my master’s son.’
LXX: ἡ παρθένος – the young woman/virgin
Exo 2:8
So the girl went and called the child’s mother. LXX: ἡ νεᾶνις – the young woman/girl
Psa 86:26
the singers in front, the minstrels last, between them maidens playing timbrels
LXX: νεανίδων – of young women/girls
Pro 30:19
the way of a man with a maiden
LXX: ὁδοὺς ἀνδρὸς ἐν νεότητι – the way of a man in youth
Sng 1:3
the maidens love you LXX: νεάνιδες – young women/girls
Sng 6:8
There are sixty queens and eighty concubines, and maidens without number.
LXX: νεάνιδες – young women/girls
Isa 7:14
a young woman LXX: ἡ παρθένος – The young woman/virgin
In all cases the word refers to a young, unmarried woman, probably a teenager. From the cultural background they would all be virgins, but that aspect may not be in focus in each instance. In four cases, the LXX used the general Greek word for a young woman, but in two places the translators chose the more specific virgin. In these two cases, a marriage is imminent, so it was important for the LXX translators to emphasize that in their understanding of the Hebrew text this young woman was clearly a virgin.
Another Hebrew word, betulah, is related. It occurs 50 times in the OT, and the LXX always translates it by the specific word for virgin (parthenos), except in 3 cases where it is used as a metaphor for a city or a group of people. There the word is translated by “daughter.”
So, comparing the two words, we can conclude from the usage that their meanings overlap.
Betulah is more specific for virgin and implies the sense of an unmarried woman (usually, but not necessarily young).
‘almah is a young woman, who is also a virgin, but this aspect may or may not be in focus, depending on context. Someone might say that if Isa 7:14 was intended to convey the sense of virgin, it should have used betulah. That kind of argument is not based on linguistic reasoning, since ‘almah can also refer to a virgin, especially if she is soon to be married.
Now, I have heard that a related word in a related language may refer to a young, married woman, but I don’t remember the details. Is that really enough to overthrow the meaning established by usage in the Hebrew Bible as well as the LXX?
Of course, translation does not mean simply substituting one word in one language with a corresponding word in another language. One needs to look at how the words weave together an overall meaning in view of its cultural context.
A very literal rendering of the Hebrew text is something like:
Therefore the Lord himself, he will give you(plural) a sign: Look/Listen! The young woman/virgin pregnant and bearing a son and she will call his name “God with us!”
The predicates here are first an adjective (pregnant), then a participle (bearing/giving birth to) and then a verb that most people understand as future (she will name him). The Hebrew verb system is complex and somewhat disputed. Does the future sense of the last verb carry over to the adjective and participle? LXX obviously decided that it did, since they translate: “Therefore Lord himself he will give you(plural) a sign: Look/Listen! The virgin will have in stomach (she will become pregnant), and she will bear a son and you (Ahaz) will call his name Emmanuel.”
One question is why there is a definite article before “virgin”. I assume it means that the reference is to a young woman, not yet married, who is known to both the speaker and hearer, possibly present. Maybe a virgin to be married to the king?
The prophecy relates to future happenings, and if the woman was already pregnant, it does not need prophetic inspiration to predict that she will give birth. This reduces it to predicting that it will be a boy. Since the word apparently does refer to a virgin, a present tense does not fit. You cannot say: “The virgin is pregnant,” even though CEV did so! King Ahaz did not witness a virgin birth. That came later with Mary as a secondary fulfilment of this small part of the prophecy. One may argue that the lack of a verb implies a present tense in English, but I find this hard to accept for two reasons. It goes against the normal meaning of ‘almah, and it implies that we know Hebrew better than the LXX translators. I know that I do not.
Those were my thoughts from a linguistic and contextual viewpoint.
