Quick answer
Так goes with a verb or adverb and never changes. Тако́й goes with a noun or adjective and agrees with it.
The division
| Russian | English | Why |
|---|---|---|
| Так хорошо́! | So good! | adverb follows |
| Тако́й хоро́ший день! | Such a good day! | adjective + noun |
| Он так говори́т. | He speaks like that. | verb |
| Он тако́й у́мный! | He’s so clever! | adjective |
| Така́я жара́! | Such heat! | noun |
With an adjective both are possible in English — “so clever” and “such a clever man” — but Russian decides by what physically follows the word: a bare adjective takes тако́й, an adverb takes так.
The useful fixed phrases
Так себе́ is the honest answer to Как дела́? — neither good nor bad. Так и есть agrees emphatically. Both are everyday and neither translates literally.
тако́й as a filler
In speech тако́й often works as an intensifier with no comparison at all: Он тако́й ми́лый! — he is so sweet. Young speakers also use it to introduce quoted speech, exactly like English “I’m like…”: А он тако́й: «Не зна́ю».
Так же or та́кже?
Two different words. Так же (two words) means “in the same way”: Он де́лает так же. Та́кже (one word) means “also”: Он та́кже прие́хал. Russians mix these up in writing constantly.
Typical mistakes
Так хоро́ший день!
Тако́й хоро́ший день!
An adjective and noun follow, so тако́й.
Тако́й хорошо́!
Так хорошо́!
An adverb follows, so так.
Check yourself
Which is correct: “Such a good day!”