Learn Hardcore German: Seine Krawatte ist so bunt, dass alle Gäste kurz lächeln. - His tie is so colorful that all the guests smile briefly.

Sein- is the possessive stem meaning his/its. It has to take an ending that matches the gender, number, and case of the noun it belongs to.

Krawatte is feminine and here it is nominative singular (it is the subject).
The correct ending for a feminine nominative singular noun is -e.

So:

masculine: sein Hut (his hat)
feminine: seine Krawatte (his tie)
neuter: sein Hemd (his shirt)
plural: seine Schuhe (his shoes)

That is why we say seine Krawatte, not sein Krawatte.

If Krawatte is feminine, why don’t we say ihre Krawatte?

The stem of the possessive (sein- vs ihr-) depends on who owns the thing, not on the gender of the thing itself.

sein- = his / its (owner is masculine or neuter: er, es)
ihr- = her / their (owner is feminine or plural: sie)

In the sentence, the tie belongs to a man (he → er), so we use the sein- stem:

owner is a man: seine Krawatte
owner is a woman: ihre Krawatte (her tie)

Both seine and ihre can describe a feminine noun, but they tell you about who owns the noun, not about the noun’s own gender.

Why is there a comma before dass?

In German, a clause starting with dass is always a subordinate clause (Nebensatz).

Rule: Main clause, comma, subordinate clause.

So we must write:

Seine Krawatte ist so bunt, dass alle Gäste kurz lächeln.

You cannot omit this comma in standard written German; it is obligatory.

Why does the verb go to the end in dass alle Gäste kurz lächeln?

In a subordinate clause introduced by dass, the finite (conjugated) verb goes to the end of the clause.

Pattern:

dass
- subject + (objects/adverbs) + verb at the end

So:

dass alle Gäste kurz lächeln
(dass – subject – adverb – verb)

Compare:

Main clause: Alle Gäste lächeln kurz. (verb in second position)
Subordinate: dass alle Gäste kurz lächeln. (verb at the end)

This verb‑final position is one of the key features of German subordinate clauses.

What exactly does so … dass express here?

The structure so … dass expresses a degree and its consequence:

so [adjective/adverb], dass [result]

In the sentence:

so bunt → such a strong degree of colorful
dass alle Gäste kurz lächeln → result of that degree

So it means roughly:

so colorful that as a result all the guests briefly smile

This is result, not purpose. For purpose (“so that they will smile”), German normally uses damit, not dass:

Er zieht eine bunte Krawatte an, damit alle Gäste lächeln.
(He puts on a colorful tie so that the guests will smile.)

Why is it bunt and not bunte in ist so bunt?

Here, bunt is used as a predicative adjective after the verb sein (ist).

Predicative adjectives in German do not take endings:

Die Krawatte ist bunt.
Die Gäste sind freundlich.
Das Essen war lecker.

Adjectives take endings only when they stand directly in front of a noun (attributive position):

eine bunte Krawatte
die freundlichen Gäste
das leckere Essen

So:

Predicate: ist bunt → no ending
Attributive: eine bunte Krawatte → with ending

What does kurz mean here? Does it mean “short” in size?

In this sentence, kurz is an adverb and means briefly / for a short moment, not “short” in length.

kurz lächeln = to smile for a brief moment

As an adjective before a noun, kurz can mean “short” in physical length or in time:

ein kurzer Rock = a short skirt
eine kurze Pause = a short break

Here it modifies the action (lächeln), not a thing, so it has the adverbial sense briefly.

Is the word order alle Gäste kurz lächeln fixed, or can I say alle Gäste lächeln kurz?

Both are possible and grammatical:

dass alle Gäste kurz lächeln
dass alle Gäste lächeln kurz

In practice, the more common version is:

dass alle Gäste kurz lächeln.

Placing kurz directly before the verb is very natural. Putting kurz after the verb can sound slightly more emphatic on the verb or more spoken-like, but the difference is subtle.

General guideline: many adverbs (like kurz, oft, gern, nie) are normally placed before the main verb inside subordinate clauses.

Why is it alle Gäste lächeln and not alle Gäste lächelt?

The subject alle Gäste is plural:

Gäste = guests (plural of Gast)

In German, the verb must agree in number with the subject. The plural form of lächeln in the present tense is:

ich lächle
du lächelst
er/sie/es lächelt
wir lächeln
ihr lächelt
sie lächeln

Since alle Gäste corresponds to sie (they), we need lächeln (3rd person plural), not lächelt (3rd person singular).

Does alle Gäste mean “all guests” or “all the guests”? Why is there no article?

Alle Gäste can usually be translated as “all the guests” in English, even though German does not use an article here.

With alle plus a plural noun, German normally drops the article:

alle Gäste = all (the) guests
alle Kinder = all (the) children
alle Bücher = all (the) books

Context decides whether you understand it as all guests in general or all the guests present in this situation. In this sentence, from context it clearly means all the guests at the event.