This is the passage in Hebrew:
וְהַנָּחָשׁ֙ הָיָ֣ה עָר֔וּם מִכֹּל֙ חַיַּ֣ת הַשָּׂדֶ֔ה אֲשֶׁ֥ר עָשָׂ֖ה יְהוָ֣ה אֱלֹהִ֑ים וַיֹּ֨אמֶר֙ אֶל־הָ֣אִשָּׁ֔ה אַ֚ף כִּֽי־אָמַ֣ר אֱלֹהִ֔ים לֹ֣א תֹֽאכְל֔וּ מִכֹּ֖ל עֵ֥ץ הַגָּֽן׃ וַתֹּ֥אמֶר הָֽאִשָּׁ֖ה אֶל־הַנָּחָ֑שׁ מִפְּרִ֥י עֵֽץ־הַגָּ֖ן נֹאכֵֽל׃ וּמִפְּרִ֣י הָעֵץ֮ אֲשֶׁ֣ר בְּתוֹךְ־הַגָּן֒ אָמַ֣ר אֱלֹהִ֗ים לֹ֤א תֹֽאכְלוּ֙ מִמֶּ֔נּוּ וְלֹ֥א תִגְּע֖וּ בּ֑וֹ פֶּן־תְּמֻתֽוּן׃ וַיֹּ֥אמֶר הַנָּחָ֖שׁ אֶל־הָֽאִשָּׁ֑ה לֹֽא־מ֖וֹת תְּמֻתֽוּן׃
When the serpent addresses the woman, it is in the common third-person plural. There are no feminine third-person plurals used here. In the two instances in which you shall die appears, it is defective (missing one instance of the letter vav [ו]) and takes an unnecessary final nun [ן]. Thus, we have תמתון (
tamutun) instead of תמותו (
tamutu). This is not the feminine plural form, which would be תָּמוֹתְנָה (
tamotna). Both defective spelling and the addition of a final nun to plural verbal forms is common in biblical Hebrew. This form is neither irregular (assuming that you mean unusual or uncommon) nor feminine plural.
The you of this dialogue is in the masculine plural, which includes the woman and her husband.