Why is there no דגש in the ת in the word "וּכְתַבְתָּ֛ם"?

Question

I am told that a בגדכפת letter takes a דגש following a closed syllable (examples of this: נִשְׁבַּ֧ע, בְּקִרְבֶּ֑ךָ and לְנֶגְדְּכֶם֙) (i.e. ends with the letter and not the vowel), so why do words like:

וּכְתַבְתָּ֛ם

כַּנְפֵ֥י

מַלְכֵ֥י

אַדְמַתְכֶ֗ם

not take a דגש in the first ת?

Answer 1

The correct verb form without the and should be: כְּתַבְתָּם – inscribe them. Although in second person plural the kamatz shortens to a sheva na, the tav won't receive a dagesh, because it doesn't come after a closed syllable. The syllabication of the word is thus כְּתַבְ-תָּם, because the sheva doesn't qualify for its own syllable. The same is the case with the -וְ prefix: even though it tranforms to -וּ, it's still not a syllable, so the syllabication remains וּכְתַבְ-תָּם however strange it seems. Therefore, we don't have a closed syllable "-וּכְ" as you write, but rather a single one as written previously, and you won't have a dagesh in the tav.

See also this French site with great examples* and an explanation of the Hebrew syllabication.

Regarding your other examples, many words with two segol in them behave in a similar way by not having a dagesh after a closed syllable. Some examples are:

singular	plural	construct
דֶּרֶךְ	דְּרָכִים	דַּרְכֵי
מֶלֶךְ	מְלָכִים	מַלְכֵי
דֶּלֶת	דְּלָתוֹת	דַּלְתוֹת
כֶּלֶב	כְּלָבִים	כַּלְבֵי
חֶסֶד	חֲסָדִים	חַסְדֵי
אֶלֶף	אֲלָפִים	אַלְפֵי
חֶרֶב	חֲרָבוֹת	חַרְבוֹת
בֶּגֶד	בְּגָדִים	בִּגְדֵי
כָּנָף	כְּנָפַיִם	כַּנְפֵי

Here the problem is that in plural the first vowel becomes a sheva or – in case of gutturals – a chataf patach. In the construct form the second vowel should be also shortened, but it is impossible, because there would be two consecutive sheva in the beginning. So the first sheva becomes a short vowel – either a patach (דרך) or a chirik (בגד), and the second vowel will be a sheva in these cases, while the dagesh is still omitted as in the plural base form.

Regarding nouns with ה ending, this ה hardens to a ת in the construct form (which was like a th sound in think in the olden days), but it seems that concerning the dagesh it was still considered an open syllable and the כ wasn't hardened (see עָרְלַתְכֶם, חָכְמַתְכֶם, גְּבוּרַתְכֶם, כְּהוּנַּתְכֶם, אֲחוּזַּתְכֶם). Although we should note that in other words like לְבַבְכֶם, אֶתְכֶם the כֶם- suffix doesn't get dagesh either.

^{* Your question would be trickier with the word בִּדְבַר discussed there, where you actually don't have a dagesh after a closed syllable.}