לָכֵן יִתֵּן ה' הוּא לָכֶם אוֹת הִנֵּה הָעַלְמָה הָרָה וְיֹלֶדֶת בֵּן וְקָרָאת שְׁמוֹ עִמָּנוּ אֵל
Therefore, G-d Himself will give you a sign: Behold, the young woman will conceive and bear a son, and she will call his name Immanuel.
This familiar verse is often mistranslated to say that a virgin will give birth and it is taken by non-Jewish sources as a messianic prophecy. It is not; it was a sign given by Isaiah to King Achaz regarding foreign affairs of the time. Rashi clarifies that the woman in question – Isaiah’s own wife – would conceive and give birth to a son that very year, which was the fourth year of the reign of King Achaz. Furthermore, she would name the child Immanuel of her own volition.