According to Brown, Driver and Briggs Lexicon (Entry #5959), Hendrickson
Publishers, Peabody, Massachusetts, 1979

• Almah – from a root meaning to be sexually mature.
• Bethulah – from a root meaning “to separate.” ibid., Entry 1330

According to : “Etymological Dictionary of the Hebrew Language”, Ernest
Klein, Carta, Jerusalem, 1987

• Almah – marriageable girl, maiden, young woman.
• Bethulah – virgin, the constellation Virgo, or either of two posts supporting
  a beam. –

For a look at these words across a broader range of Hebrew Literature: see
“Dictionary of the Talmud,” by Marcus Jastrow, Ph.D.,

Since Isaiah is not available for comment, we can only guess at why he chose
the one word instead of the other.

Some observations:

1. As to whether it is legitimate to read the word “almah” as virgin, it is
   possible that in Isaiah’s day the word almah was the usual word for a
   virgin. Evidently the translators of the Septuagint, who were much closer
   to Isaiah’s time than modern scholars, saw fit to translate it parthenos –
   virgin. The author of Matthew apparently also interpreted the word as
   meaning virgin.
2. 
   When taken in context, it is clear that on the surface Isaiah was referring
   to an event in his own time. If this original woman had been a virgin in
   the strictest sense, the miracle of Messiah’s birth would not have been
   unique. Nevertheless, the concept of virgin giving birth to “God with Us”
   provided a glimpse of the coming Messiah, and the author of Matthew
   evidently picked up on this.