In the phrase "Qui l'eût cru" from the French song La Madrague, the mood is the conditional and the tense is the past. The phrase can be translated to "Who would have believed it?" in English. The construction "eût cru" is a past conditional form of the verb "croire" (to believe) using the past subjunctive auxiliary verb "eût" (from the verb "avoir") followed by the past participle "cru". This form is more formal and antiquated, as you mentioned, it is inspired by El Cid, a play by Corneille. In modern French, the past conditional is typically formed using the auxiliary verb "aurait" (from the verb "avoir") followed by the past participle, as in "Qui l'aurait cru?" and "Qui l'aurait dit?" But we use the simple past a lot in literature. You can also use "Qui l'eût cru?" as a daily question, when you are surprised of something.

This is relatively antiquated language that is almost certainly drawn from and inspired by El Cid, a play by Corneille. When the hero (Rodrigue) and the damsel (Chimène) lament that life is tearing them apart and their happiness will be short lived. Chimène: Qui l’eût cru? Rodrigue: Qui l’eût dit? Who would have believed it? Who would have said it? In modern French, the most common way of saying this would be: Qui l'aurait cru? Qui l'aurait dit?

"Qui l'eût cru" is a phrase that means "Who would have thought?" or "Who could have imagined?" It is a rhetorical question expressing surprise, wonder or disbelief about something that has happened. The tense used in this phrase is the conditional past, also called the "second conditional" or "conditional perfect" in English grammar. In French, it is formed with the auxiliary verb "avoir" or "être" in the conditional present, followed by the past participle of the main verb. The conjugation of "avoir" in the conditional present is "aurais", "aurais", "aurait", "aurions", "auriez", and "auraient". In the song La Madrague, the full line is "Qui l'eût cru qu'aujourd'hui je vive simplement", which means "Who would have thought that today I would live simply". The use of the conditional past tense suggests that the speaker is surprised by their current situation, as it is not what they would have expected in the past.

Oups ! Merci pour la correction ! ;)