Если дрожжи свежие, кекс и булка пахнут лучше, чем вчера.

In Russian, the verb to be is usually omitted in the present tense.

So:

• дрожжи свежие = the yeast is fresh / the yeast are fresh
• literally: yeast fresh

This is completely normal. In past and future, forms of быть do appear:

• Дрожжи были свежие = The yeast was fresh
• Дрожжи будут свежие = The yeast will be fresh

So the missing are is not a mistake; it is standard Russian grammar.

Дрожжи is a plural-only noun in Russian. That means it normally behaves grammatically like a plural word, even though in English yeast is usually treated as a mass noun.

Because дрожжи is plural, words agreeing with it are also plural:

• свежие дрожжи
• дрожжи свежие
• дрожжи были свежие

So Russian says something closer to yeasts grammatically, even though the meaning is just yeast.

It agrees with дрожжи, which is grammatically plural.

Compare:

• свежий кекс = fresh cake masculine singular
• свежая булка = fresh bun/loaf feminine singular
• свежие дрожжи = fresh yeast plural

So in Если дрожжи свежие, the adjective must be plural: свежие.

You may also encounter the short-form predicate дрожжи свежи, but свежие is very natural and neutral here.

Because кекс и булка is a compound subject: cake and bun. Two nouns joined by и normally take a plural verb.

So:

• кекс пахнет = the cake smells
• булка пахнет = the bun smells
• кекс и булка пахнут = the cake and the bun smell

That is why Russian uses пахнут, the 3rd person plural form.

They are all in the nominative case because they are subjects.

• Если дрожжи свежие → дрожжи is the subject of the subordinate clause.
• кекс и булка пахнут лучше → кекс and булка are the subjects of the main clause.

A good clue is that these nouns are the things being described or doing the action, not objects receiving an action.

Yes. Лучше is the comparative form of хорошо and works here as an adverb: smell better.

Russian commonly uses:

• хорошо пахнуть = to smell good
• плохо пахнуть = to smell bad
• лучше пахнуть / пахнуть лучше = to smell better

So лучше describes how they smell, not the nouns themselves.

It means than yesterday, but more fully the idea is:

• пахнут лучше, чем вчера = smell better than they did yesterday

Russian often leaves out words that are understood from context. So after чем there is an implied idea such as:

• чем пахли вчера
• or simply чем вчера

This kind of omission is very common and natural.

Because Если дрожжи свежие is an если clause, a subordinate clause meaning if the yeast is fresh.

Russian separates that clause from the main clause with a comma:

• Если дрожжи свежие, кекс и булка пахнут лучше, чем вчера.

This is like English punctuation in many if sentences:

• If the yeast is fresh, the cake and the bun smell better.

Because чем introduces the comparison: better than yesterday.

In sentences like this, Russian normally writes a comma before чем, especially when the second part can be understood as a shortened clause:

• лучше, чем вчера
  = better than (they smelled) yesterday

So that comma is standard here.

Yes. Russian word order is more flexible than English word order.

This sentence could be rearranged, for example:

• Кекс и булка пахнут лучше, если дрожжи свежие.
• Если свежие дрожжи, кекс и булка пахнут лучше.
• Кекс и булка лучше пахнут, если дрожжи свежие.

The basic meaning stays similar, but the emphasis changes a bit.

The original sentence is very natural and neutral.

Если is the normal word for if in ordinary conditional sentences.

So:

• Если дрожжи свежие... = If the yeast is fresh...

There is no need for a word meaning then in the main clause, although Russian can sometimes use то for emphasis:

• Если дрожжи свежие, то кекс и булка пахнут лучше.

That version is also correct, but то is optional here.

• кекс usually means a cake, often a small loaf cake, sponge cake, or something similar.
• булка can mean a bun, roll, or sometimes a small loaf depending on context.

So the exact English translation may vary a little depending on what kind of baked goods the speaker has in mind.

Russian has no articles. So nouns like кекс and булка can mean:

• a cake / the cake
• a bun / the bun

Context tells you which is meant.

The same is true for many Russian sentences. Learners often want a direct equivalent of a or the, but Russian usually does not mark that distinction explicitly.