Pronunciation of "поздно" (late)

Question

The word поздно (late) is pronounced as [ˈpoznə]. So the д is not spoken. I do not understand why the д vanishes here in this case. Which grammar rule tells me this?

Answer 1

Quite a few consonant clusters have silent letters in them, e.g.:

• здн (пра́здник, пра́здновать, по́здний, по́здно, бе́здна, звёздный), silent д.

• стн (ле́стница, до́блестный, захолу́стный, я́ростный), silent т.

• стл (счастли́вый, со́вестливый, уча́стливый), silent т.

• стс (раси́стский, национали́стский), silent т.

Some sources claim a silent т in the нтс / ндс cluster but Avanesov's dictionary recommends the pronunciation [нц^с]:

• ндс (голла́ндский, шотла́ндский, финля́ндский).

• нтс (гига́нтский, парла́ментский, аге́нтство).

These words also have silent letters:

• со́лнце, се́рдце, пожа́луйста (but not пожа́луйте)

• здра́вствуй, чу́вство, but not я́вственный, нра́вственный or баловство́ (as pointed out by @Yellow_Sky below)

In russiangram.com, I used the following regex to cover the most common cases:

(?<left>[сзн])(?<silent>[дт])(?<right>[нлс])

Answer 2

Grammar rules usually don't tell anything about pronunciation at all. You should just know that there are certain consonant clusters where it happens all the time (muted letter is in bold): солнце, здравствуй, сердце, чувство and where you can hear both forms, like, say, in "гигантский".

It's like asking - which grammar rules make some English speakers pronounce "often" with or without t.

In Russian, though, we usually can answer the question of why we should write the muted consonant after all. Usually because in a different form this consonant is pronounced. Such a word is called "проверочное слово". For instance:

• солнечный
• сердечный
• гигант
• опоздать (therefore "поздно")

Usually, when we don't have such words, we don't have a version with muted consonant as well. It's not a rule set in stone however, for some words, like, for instance "лестница" or "чувство" - we won't find any other modern Russian word justifying such spelling.

So, memorize. Just memorize - as I always advise anyone learning any language.