Quote Originally Posted by it-ogo View Post
Either way is right. Namely both of them. Officially the street is called "Невский проспект" but in colloquial speech people usually skip the word "проспект" and operate the rest as a "soft adjective".
I believe the original question was why "по Невскому" is spelled with "-ому" and not "-ему" if it has "-ий" in its dictionary form.

Quote Originally Posted by zorro View Post
Говорят, что Невский проспект--это одна из самых красивых и интересных улиц в мире. This makes me think that Невский is a soft adjective. Then there's another line: Мне очень хочется пойти погулять по Невскому проспекту. I would expect the spelling Невскему for a soft adjective. What am I missing here?
Yes, you are probably missing something.
Sounds "к, г, х" do not behave the usual way regarding their hard and soft versions.

On the one hand, you are right.

If the dictionary form ends in "-ой" (stressed) or "-ый" (unstressed), this is a hard-stem adjective, and it has "hard" endings in all its forms:

(Stressed vowel is red. I only mark it in the dictionary form since the stress in full adjectives is always fixed):

прямой (straight) - прямого, прямому, прямым, о прямом; прямая - прямое - прямые etc. (I do not list all the forms here)
острый (sharp) - острого, острому, острым, об остром; острая - острое - острые etc.

If the dictionary form ends in "-ий" (always unstressed), this is a soft-stem adjective, and it has "soft" endings in all its forms:

синий (blue) - синего, синему, синим, о синем; синяя - синее - синие etc.

The absolute majority of soft-stem adjectives end in "-ний": синий, крайний, дальний, летний, зимний etc. There are lots of them.
And there is only one soft-stem adjective which does not end in "-ний". It is "карий" ("brown", only applicable to the color of eyes).

Theoretically, adjectives ending in "-чий", "-щий" also belong to the soft-stem group, as "горячий" (hot) or "вещий" (prophetic). But "ч" and "щ" are not contrasted (they are primordially soft). And there are also some spelling rules which apply to "ч" and "щ" (we never write "я" and "ю" after them, "а" and "у" are written instead): горячий, горячее, горячие - but горячая, горячую; вещий, вещее, вещие - but вещая, вещую. So, "ч" and "щ" are somewhat a special case.

No other soft-stem adjectives exists (we do not have any adjectives which end in "-бий, -вий, -дий, -зий, -лий etc. ...").
Note: лисий "(related to) fox", коровий "(related to) cow" and other similar cases are not true adjectives. They are so-called possessive adjectives, and they have their own declension type which does not match the common declension pattern of true adjectives. So, we do not consider them here.

On the one hand, this is not the case when the final stem consonant is one of "к, г, х" sounds.

Most of the Russian consonants exist in two varieties: hard and soft (palatalized). They can be contrasted in virtually any position: before a vowel (мал - мял, ров - рёв, лук - люк, был - бил), before a consonant (полка - полька, горка - горько) and word-finally (вес - весь, кров - кровь, угол - уголь, стан - стань). But all those consonants are always soft before [е]: лес, вес, степь, река etc. Syllables like "мэ, тэ, дэ" etc. never occur in Russian native words, there are only a few loanwords which have them.

The sounds "к, г, х" also exist in hard and soft varieties. But they are never opposed! Those three consonants are somewhat "special": their hardness or softness is easily predicted, and it is just defined by the sound which comes after.
Note. As it is a matter of phonetics, I will use phonetic representations of Russian vowels below.
Russian only has 5 vowel phonems ("basic" vowels or "underlying" vowels). They are [a], [e], [ i ], [o], [u]. But in writing each of them can be represented by two different letters depending on the preceding consonant ("а, э, ы, о, у" indicate the preceding consonant is hard, and "я, е, и, ё, ю" indicate the preceding consonant is soft).
1. The sounds "к, г, х" are always hard word-finally: бык, друг, вздох - there is no Russian word which ends in "кь, гь, хь".
2. They are always hard before another consonant: окно, игла, пихта. No Russian word ever contains "кь, гь, хь" combinations!
3. They are always hard before vowels [a], [o], [u]: карта, кот, куб, галька, год, губа, хата, хобот, хуже. The combinations "кя, гя, хя, кё, гё, хё, кю, гю, хю" are impossible in native Russian words. Although they can sometimes happen in some loan words, but it is a very rare case. An example of such a word is "маникюр" (manicure). Other examples are Гюльчатай (oriental women's name), Кёльн (Köln - a city in Germany), Гёте (Goethe - a German philosopher) etc.
4. However, those three consonants are always soft before vowels [i] and [e]: кит, пакет, гиря, Сергей, хитрый, хек. The combinations "кы, гы, хы, кэ, гэ, хэ" are impossible in native Russian words. Some foreign proper names can contain them like Кыргызстан (Kyrgyzstan) which is more commonly known in Russia as Киргизия (this example demonstrates that pronouncing "ки" and "ги" is much more natural for Russians than alien "кы" and "гы").

To sum it up, the rule is: "к, г, х" are always palatalized before [i] and [e] (represented as "ки, ги, хи" and "ке, ге, хе" in spelling). And they always stay hard in any other position.

Now, how it affects the adjectives whose final stem consonant is "к", "г" or "х".
If the stress is on the ending, then we have a dictionary form which ends in "-кой", "-гой" or "-хой".
такой (such) - такого, такому, таким, о таком; такая - такое - такие etc.
другой (other, different) - другого, другому, другим, о другом; другая - другое - другие etc.
плохой (bad) - плохого, плохому, плохим, о плохом; плохая - плохое - плохие etc.

Note that the ending is "hard" if it contains [a], [o], [u] (такой, другой, плохой; такая, другая, плохая; такую, другую, плохую) but it "automatically" becomes "soft" if it contains [i] (таким, другим, плохим) - this is one of basic phonetic principles in Russian.

If the stress is on the ending, then we have a dictionary form which ends in "-кий", "-гий" or "-хий".
горький (bitter) - горького, горькому, горьким, о горьком; горькая - горькое - горькие etc.
строгий (strict) - строгого, строгому, строгим, о строгом; строгая - строгое - строгие etc.
тихий (quiet, silent) - тихого, тихому, тихим, о тихом; тихая - тихое - тихие etc.

Note that the same principle applies: hard endings with [a], [o], [u] and soft endings with [i]. (Adjectives do not have endings with [e], so I do not mention it here).

And it does not only work with adjectives!

It applies to nouns, verbs and any other words which contain "к, г, х". This is a general pronunciation principle.

Check some examples with nouns:
губа "lip" (sg) - губы (pl); корова "cow" (sg) - коровы (pl); коза "she-goat" (sg) - козы (pl) - hard stem;
пуля "bullet" (sg) - пули (pl); баня "bath-house" (sg) - бани (pl); буря "storm" (sg) - бури (pl) - soft stem;
but:
рука "hand, arm" (sg) - руки (pl); нога "foot, leg" (sg) - ноги (pl), блоха "flea" (sg) - блохи (pl).
You can call it "mixed" stem if you like But the reason for it is the phonetic principle which dictates when "к, г, х" are hard and when they are soft.

There is only one exception in Russian!

Finally, we do have an exception. It is the verb "ткать" (to weave). Examine its forms:

я тку, ты ткёшь, он ткёт, мы ткём, вы ткёте, они ткут.
The forms "ткёшь, ткёт, ткём, ткёте" contain a soft "к" followed by [o]: кё, which does not exist in any other word in standard literary Russian.

And the verbal adjective is "ткя" (while weaving). It contains a soft "к" followed by [а]: кя. It does not exist in any other word in standard literary Russian either.