Does the comparative form of adjectives need to have declension?

Question

For example, лу́чше seems to be used for all genders (and cases?) When is it necessary to use the fully declined versions (лу́чший, лу́чшего etc.)

Answer 1

This is a rather complicated topic, so I'll start with an oversimplification.

Лучший is not a form of лучше, but rather both of them are forms of хороший.

Лучше is a simple comparative of хороший, whereas лучший is usually a simple superlative of хороший.

The compound comparative and superlative variants are более хороший and самый хороший respectively.

Simple comparatives usually do not change with the cases, genders, or numbers because the word is preoccupied with displaying its comparative status. However, the compound comparative does change appropriately, as the second part of it is the base adjective.

Unfortunately, лучший can also be used sometimes as a comparative of хороший as well, which further complicates things. There is hardly any semantic difference between comparative лучше and лучший, but лучше is much more common as a comparative since it avoids possible confusion of comparative лучший with a superlative лучший:

• Я строю лучший дом (my current house is unsatisfying, so I am building a better one)

• Я строю лучший дом (this house is going to be the best)

As for when you should use them, it is a rather straightforward matter.

You use the base adjective to describe an attribute of an object:

• хороший (good)
• красный (red)
• быстрый (fast)

You use either the simple or compound comparative to convey that an object possesses more of the attribute than another object (or objects):

• более хороший/лучше/лучший (better)
• более красный/краснее (redder)
• более быстрый/быстрее (faster)

You use the simple or compound superlative to convey that an object possesses the most of the attribute compared to the rest:

• самый хороший/лучший (the best)
• самый красный/краснейший (the reddest)
• самый быстрый/быстрейший (the fastest)

Finally, just to complicate things more (:D) I should also mention that лучше can be a simple comparative of both the adjective хороший and the adverb хорошо. Furthermore, it has several other uses derived from these:

• Ситуация теперь лучше (predicative: conveys improvement)
• Лучше запиши, а то забудешь (either identified as an adverb or a particle: conveys preferable outcome or an attempt to convince someone)