Seksen yetmişten fazladır.

Because Turkish uses the ablative suffix here: -den / -dan / -ten / -tan.

In comparison sentences, this suffix often does the job that English than does.

So:

• yetmiş = seventy
• yetmişten = than seventy

In this sentence, yetmişten means than seventy, not literally from seventy in the English sense, even though the ablative suffix originally has a from meaning.

In sentences like this, yes.

A very common Turkish comparison pattern is:

X, Y-den fazla
= X is more than Y

So:

• Seksen yetmişten fazladır.
• literally: Eighty is more from seventy
• naturally: Eighty is more than seventy

So while -ten is not always equal to than, it often works that way in comparisons.

Fazla means more, too much, or extra/excess depending on context.

In this sentence, it means more:

• yetmişten fazla = more than seventy

So fazla is the key comparison word here.

With numbers and quantities, fazla is very common.

The -dır part is a copular suffix. It adds the sense of is / are and can also sound a bit more:

• formal
• definite
• explanatory
• textbook-like

So:

• Seksen yetmişten fazla. = common, natural, everyday
• Seksen yetmişten fazladır. = more formal or emphatic

Both are correct.

Yes. Very often.

In the present tense, Turkish often uses no separate word for is/am/are in sentences like this.

That means:

• Seksen yetmişten fazla. literally has no separate word for is
• but it still means Eighty is more than seventy

If the speaker wants a more formal or emphatic style, they can add -dır:

• fazladır = is more

Because of two sound rules in Turkish:

1. Vowel harmony
2. Consonant harmony / devoicing

The last vowel in yetmiş is i, so the suffix uses the front-vowel form: -de/-den or -te/-ten.

Also, yetmiş ends in ş, which is a voiceless consonant, so the suffix starts with t rather than d.

That gives:

• yetmiş + ten = yetmişten

Yes, but Turkish is somewhat flexible.

The most natural order is:

• Seksen yetmişten fazladır.

This follows a common Turkish pattern where the predicate comes at the end.

Very roughly:

• Seksen = subject
• yetmişten = comparison phrase
• fazladır = predicate

Turkish can move things around for emphasis, but this version is the standard and safest one for learners.

Yes, absolutely.

That is probably the more everyday version.

Compare:

• Seksen yetmişten fazla. = natural, conversational
• Seksen yetmişten fazladır. = more formal, written, or emphatic

So if you are speaking casually, dropping -dır is very normal.

Yes, in a way.

Number words in Turkish can stand on their own and function like noun-like elements in a sentence.

Here:

• Seksen acts as the subject: eighty
• yetmişten is the thing being compared against: than seventy

So even though they are numbers, they can behave like full sentence parts.

Yes, and that is also correct.

There is a slight nuance:

• fazla = more
• büyük = big / greater

So:

• Seksen yetmişten fazladır. = Eighty is more than seventy.
• Seksen yetmişten büyüktür. = Eighty is greater than seventy.

In mathematics, büyüktür can sound especially natural because English also often says greater than.

A little, yes.

The main reason is -dır.

Without it:

• Seksen yetmişten fazla.

With it:

• Seksen yetmişten fazladır.

The version with -dır sounds a bit more like:

• a textbook
• a definition
• a general statement
• something written rather than casual speech

So learners should understand it, but also know that everyday Turkish often drops -dır.

You would use değil.

For example:

• Seksen yetmişten fazla değil. = Eighty is not more than seventy.
• Seksen yetmişten fazla değildir. = same meaning, but more formal

So değil is the normal way to negate this kind of sentence.