Learn Hardcore French: Selon Marie, le pain chaud est bon le matin. - According to Marie, warm bread is good in the morning.

No. In French, placing a comma after an introductory prepositional phrase (like Selon Marie) is common to indicate a pause and improve readability, but it is not strictly required. It helps separate the introductory phrase from the main clause.

Why is selon used instead of d’après?

Both selon and d’après translate to according to
Selon is neutral and widely used in both written and spoken French
D’après feels a bit more conversational or informal and often appears in spoken language or journalistic contexts

Why do we say le pain chaud instead of du pain chaud?

Le pain chaud with the definite article expresses a general truth: hot bread (in general) is good in the morning
Du pain chaud (partitive article) would mean some hot bread, focusing on an unspecified quantity rather than the concept of hot bread as a whole

Why is the adjective chaud not chaude?

French adjectives agree in gender and number with the nouns they modify. Pain is masculine singular, so the adjective takes its masculine singular form: chaud.

Why is bon masculine singular instead of feminine or plural?

Because bon agrees with its subject, le pain chaud, which is masculine singular. If you were talking about les viennoiseries chaudes, you would say elles sont bonnes.

Why is there a le before matin, and could we omit it?

Time-of-day expressions in French generally require the definite article
Le matin means in the morning. Omitting le would make it sound unnatural, with a few rare exceptions (e.g., demain matin)

Can we move le matin to the beginning of the sentence?

Yes. French word order is flexible for adverbial phrases:
Le matin, selon Marie, le pain chaud est bon.
Using commas helps separate the time phrase from the rest of the sentence.