Learn Italian: Dobbiamo celebrare San Marco perché è un gran santo. - We must celebrate Saint Mark because he is a great saint.

The title santo drops its final syllable and becomes San when it comes directly before a masculine name starting with a consonant. The main exceptions are names starting with s plus another consonant (like Santo Stefano) or z (like Santo Zeno).

Why does the sentence use gran santo instead of grande santo?

Just like santo, the adjective grande (great) frequently drops its final syllable to become gran before singular nouns. This rule applies before nouns starting with most consonants, including the s plus vowel in santo.

Since santo can truncate to San, why doesn't it do that at the end of the sentence?

The word santo only shortens to San when it acts as a title placed right in front of a specific person's name. In the phrase gran santo, it is simply a regular noun meaning 'a saint', so it keeps its full form.

Do we need a preposition like a or di between dobbiamo and celebrare?

No. Dobbiamo comes from dovere (must or to have to), which is a modal verb. Modal verbs connect directly to the infinitive form of the main verb without needing any prepositions in between.