La laitue est plus fraîche aujourd’hui, mais les radis de ce marché sont meilleurs que d’habitude.

Because French articles have to match the noun’s gender and number.

• laitue is feminine singular → la laitue
• radis is plural here → les radis

If you wanted the singular, it would be le radis.

This is one of the first things English speakers notice: French nouns have grammatical gender, so you have to learn the noun together with its article.

Because fraîche agrees with laitue, and laitue is feminine singular.

The base adjective is:

• masculine singular: frais
• feminine singular: fraîche

So:

• Le pain est frais.
• La laitue est fraîche.

This is adjective agreement: the adjective changes form to match the noun.

For the same reason: agreement.

Even after plus, the adjective still has to agree with the noun it describes.

• la laitue → feminine singular
• therefore → plus fraîche

Compare:

• Le radis est plus frais.
• La laitue est plus fraîche.

So plus does not stop the adjective from changing form.

Because bon has an irregular comparative in standard French.

Just as English has good → better, French has:

• bon → meilleur

So the normal pattern is:

• bon = good
• meilleur = better

Examples:

• Ce fromage est bon.
• Ce fromage est meilleur aujourd’hui.

You may sometimes hear plus bon in very specific contexts, but for most learners, meilleur is the correct choice.

Because it agrees with les radis, which is masculine plural.

Forms of meilleur:

• masculine singular: meilleur
• feminine singular: meilleure
• masculine plural: meilleurs
• feminine plural: meilleures

Since the subject is les radis, you need meilleurs.

Plus ... que is the basic French pattern for more ... than.

In this sentence:

• plus fraîche = fresher
• que d’habitude = than usual

So the structure is:

• plus + adjective + que

Examples:

• plus grand que = taller than
• plus rapide que = faster than
• plus fraîche que d’habitude = fresher than usual

French uses que where English uses than.

Because d’habitude is a fixed expression meaning usually or as usual, and after a comparative it means than usual.

So:

• meilleurs que d’habitude = better than usual

This is a very common French expression. You should learn d’habitude as a chunk.

Examples:

• Il est plus calme que d’habitude.
• Le café est plus fort que d’habitude.

Here de means something like from or of/from this market.

• les radis de ce marché = the radishes from this market / the radishes at this market

It connects the radishes to a specific market.

French often uses de where English might use several different prepositions, depending on context.

Because marché is masculine singular and starts with a consonant.

The demonstrative adjectives are:

• ce
  • masculine singular before a consonant
    → ce marché
• cet
  • masculine singular before a vowel or silent h
    → cet hôtel
• cette
  • feminine singular
    → cette laitue
• ces
  • plural
    → ces radis

So ce marché is the correct form.

Because the verb être has to agree with the subject in number.

• la laitue = singular → est
• les radis = plural → sont

So:

• La laitue est...
• Les radis sont...

This is the same idea as English is vs are.

Good question: radis is one of those nouns whose singular and plural are spelled the same.

• singular: le radis
• plural: les radis

In the sentence, you know it is plural because of:

• the article les
• the verb sont
• the adjective meilleurs

So even though the noun itself does not change spelling, the rest of the sentence shows that it is plural.

Yes, it could.

In the sentence, aujourd’hui comes after plus fraîche, which sounds very natural:

• La laitue est plus fraîche aujourd’hui...

But French is flexible with time expressions. You could also say:

• Aujourd’hui, la laitue est plus fraîche...

Both are correct. The version in your sentence is smooth and conversational.

A few of them can be hard for English speakers:

• laitue → roughly lay-tuu
• fraîche → roughly fresh
• aujourd’hui → roughly oh-zhoor-dwee
• radis → roughly ra-dee
• meilleurs → roughly may-yur
• habitude → roughly a-bee-tuud

A few notes:

• The final -s in radis and meilleurs is not pronounced here.
• fraîche has a final written -e, but that e is not pronounced.
• French r is not like English r; it is produced farther back in the throat.

French uses definite articles more often than English.

So French often says:

• la laitue
• les radis

where English might sometimes say just lettuce or radishes, depending on context.

In this sentence, the definite articles make the vegetables sound like specific produce being discussed, or produce understood from the situation. That is very normal in French.