When the ת Moves: הסתכל, השתמש, הצטלם, הזדקן

Build the root ס־כ־ל into hitpa'el and the machinery gives you הִתְסַכֵּל. Nobody says that. The word is *הִסְתַּכֵּל — the ת of the prefix and the ס of the root have swapped places. This is the one genuinely irregular thing hitpa'el does, it affects a large and very common group of verbs, and most courses teach half of it.

The condition: a sibilant first radical

He.Wikipedia's article on שיכול עיצורים (consonant metathesis) states the rule and names the set: the swap happens when the first root letter is one of the hissing consonants — צ (including צ׳), ס, ש, ז (including ז׳) or ג׳. After the swap, two further adjustments may follow: voicing assimilation, turning the ת into ד, and emphatic assimilation, turning it into ט.

Read that set carefully: ש stands for both שׁ and שׂ, and the geresh letters are completeness rather than vocabulary — צ׳ and ז׳ follow their base letters, and ג׳ heads no hitpa'el verb you are likely to meet. That leaves five rows worth learning, and between them only three outcomes, which divide by sound rather than by letter shape.

The שׂ row is worth a line of its own, because almost nothing mentions it. Pealim carries לְהִשְׂתַּכֵּר "to earn" on the root שׂ־כ־ר and לְהִשְׁתַּכֵּר "to get drunk" on שׁ־כ־ר as two separate entries — a genuine minimal pair distinguished by one dot, and in unpointed writing not distinguished at all. Both come out as משתכר.

Why ט and ד, and not something arbitrary

These two look like memorisation until you stop reading letters and start listening.

ז is voiced, and this change you can still hear. Your vocal cords are running for the z, and the ת that follows is voiceless — so the cluster stops fighting itself and the ת becomes ד, the voiced partner of ת. hitzakén would need the voicing switched off and straight back on inside one cluster; hizdakén would not, and hizdakén is what Hebrew says.

צ was emphatic, and that change is now only visible. צ was historically a hardened s, whose partner in the same class was ט, the hardened t — so beside צ the ת was written ט to match. But ט and ת have merged in Israeli Hebrew: both are an ordinary [t]. You say hitstalém whichever letter is written, so this half of the rule survives purely as a spelling rule. That is precisely why it gets forgotten.

In both cases the ת is agreeing with its new neighbour. ס, שׁ and שׂ are voiceless and unemphatic already, so there is nothing for the ת to agree with and it stays exactly as it is.

The swap is permanent — it runs through every cell

Once a verb has metathesised, it has metathesised for good. There is no cell of the paradigm where the original order comes back — so the endings you learned on the hitpa'el present and the hitpa'el past attach to the reordered stem without any further adjustment.

A different mechanism that gets filed with this one

When the first root letter is ד, ט or ת, nothing swaps. Instead the prefix ת is swallowed by the root letter, which doubles to compensate — total assimilation, not metathesis. The two are constantly lumped together because both produce a word where the expected תְ has vanished.

Watch the spelling: these are the hitpa'el verbs that are written with a yod in ktiv male — הידבר, היטהר, היתמם, תיתמם — because the prefix vowel ends up in a closed syllable in front of a doubled letter. That yod is the visible trace of the swallowed ת.

And everywhere else, nothing happens at all

This is the reassuring half. Non-sibilant roots — the large majority — keep the ת exactly where the paradigm puts it, and hitpa'el behaves like a completely regular binyan.

The sibilant is only ever the first root letter for these purposes. הִתְרַגֵּשׁ has a שׁ in it and does not swap, because the שׁ is at the end; הִתְלַבֵּשׁ is the same. Check position one and nothing else.

Common Mistakes

Every error below is the same error underneath: assembling the word from the bare root instead of from the finished stem. All four are forms learners genuinely produce, and none of them is a Hebrew word.

1. ס — leaving the ת in front. Shown here in the infinitive, since that is the form you build from a root you have just looked up.

2. שׁ — the same thing in the past. The reordering is not a feature of one tense; it is in the stem.

3. צ — no swap, or a swap without the ט. Both halves of this rule have to fire, and either one can go missing.

4. ז — missing the ד.

Key Takeaways

• Metathesis is triggered by a sibilant first root letter. He.Wikipedia's שיכול עיצורים names the set as צ (with צ׳), ס, ש, ז (with ז׳) and ג׳ — of which five carry everyday verbs, once ש is split into שׁ and שׂ.
• ס, שׁ, שׂ: the letters simply swap — הִסְתַּכֵּל, הִשְׁתַּמֵּשׁ, הִשְׂתָּרֵעַ.
• צ: swap plus emphatic assimilation, ת → ט — הִצְטַלֵּם, הִצְטָרֵף, הִצְטַנֵּן.
• ז: swap plus voicing assimilation, ת → ד — הִזְדַּקֵּן, הִזְדַּמֵּן, הִזְדָּרֵז.
• The reason is agreement, not arbitrariness: the ת takes on the voicing of ז and the emphasis of צ, and has nothing to take on from ס, שׁ and שׂ. Since ט and ת have merged in Israeli Hebrew, only the ז rule is audible — the צ rule is now a spelling rule.
• שׂ־כ־ר and שׁ־כ־ר give הִשְׂתַּכֵּר "earn" beside הִשְׁתַּכֵּר "get drunk" — identical in ktiv male, both משתכר.
• The swap persists through every cell: הִשְׁתַּמַּשְׁתִּי, מִשְׁתַּמֵּשׁ, אֶשְׁתַּמֵּשׁ, לְהִשְׁתַּמֵּשׁ.
• A first radical ד, ט or ת is a different mechanism — total assimilation, spelled with a yod in ktiv male: הידבר, היטהר, היתמם.
• It is confined to hitpa'el: pa'al לִתְסוֹס and hif'il הִתְסִיס keep their ת+ס intact.
• Non-sibilant roots never do any of this: התלבש, התרגש, התקרר. Only position one counts.