Re Isaiah 7:14, in his commentary on The Prophecy of Isaiah (pp. 84-85) J. Alec Motyer, writes:
"The translation virgin ('alma) is widely disputed on the ground that the word means only 'young woman' and that the technical word for 'virgin' is beṯûla. Of the nine occurrences of 'alma those in 1 Chronicles 15:20 and the title of Psalm 46 are presumably a musical direction but no longer understood. In Psalm 68:25; Proverbs 30:19; and Song of Solomon 1:3 the context throws no decisive light on the meaning of the word. In Genesis 24:43 and Exodus 2:8 the reference is unquestionably to an unmarried girl, and in Song of Solomon 6:8 the alamot, contrasted with queens and concubines, are unmarried and virgin. Thus, wherever the context allows a judgment, 'alma is not a general term meaning 'young woman' but a specific one meaning 'virgin'. It is worth noting that outside the Bible, so far as may be ascertained, 'alma was 'never used of a married woman'."
"Genesis 24 is particularly important as providing a direct comparison of 'alma and beṯûla. Abraham's servant's prayer (24:14) is couched in terms of a 'girl' (naara) who is to marry Isaac. In verse 16 the approaching Rebekah is described as female (naara), of marriageable age (beṯûla) and single ('no man had ever lain with her'). The qualifying words indicate that by itself beṯûla is not specific. In the light of this accumulating knowledge of Rebekah, verse 43 finally describes her as 'alma, which is clearly a summary term for 'female, marriageable, unmarried'. There is no ground for the common assertion that had Isaiah intended virgo intacta he would have used beṯûla. 'alma lies closer to this meaning than the other word. In fact, this is its meaning in every explicit context. Isaiah thus used the word which, among those available to him, came nearest to expressing 'virgin birth' and which, without linguistic impropriety, opens the door to such a meaning."