Learn Italian: Dovresti respirare profondamente ogni mattina. - You should breathe deeply every morning.

Dovresti is the present conditional form of dovere (to must/to have to). While devi means "you must" and sounds like a strict command, using the conditional dovresti softens it to "you should." We use this form to give polite advice.

Do I need a linking word like a or di between dovresti and respirare?

No. Dovere is a modal verb (a helper verb), and in Italian, modal verbs are always followed directly by the infinitive (the dictionary form) of the main verb. So you just say dovresti respirare without any prepositions in between.

How do you form adverbs like profondamente?

To make an adverb out of an adjective, you usually take the feminine singular form of the adjective (in this case, profonda instead of the masculine profondo) and add the ending -mente. This is exactly like adding "-ly" to an adjective in English.

Could I say tutte le mattine instead of ogni mattina?

Yes, absolutely! Ogni mattina means "every morning" and is always followed by a singular noun with no article. Tutte le mattine means "all the mornings" and requires the plural noun with the definite article (le). Both are very common and mean the exact same thing.