Learn Arabic (Modern Standard): كان يجب أن يرفض المدير هذه الصفقة لأن الخسارة المالية كانت كبيرة. - The manager should have rejected this deal because the financial loss was big.

يجب أن literally means 'it is necessary that'. To make it past tense ('it was necessary that' or 'should have'), we add كان (it was) at the beginning. Notice that يجب stays in the present tense; the كان is what shifts the entire obligation into the past.

Why is the verb يرفض (reject) in the subjunctive mood here?

The subjunctive mood is triggered by the particle أن (that) immediately preceding it. For a regular present tense verb like this, the final vowel changes from a damma (u) to a fatha (a), making it يرفضَ (yarfuḍa).

Could we put المدير (the manager) before the modal verb, like كان المدير يجب أن يرفض?

No, because يجب is an impersonal verb meaning 'it is necessary'. The manager is the subject of the rejecting (يرفض), not the subject of the necessity. Therefore, المدير must go after يرفض as its subject, or at the very beginning of the whole sentence as a topic.

Why do we use لأن for 'because' here instead of بسبب?

We use لأن when the 'because' introduces a full sentence or clause (here, 'the financial loss was big'). بسبب means 'because of' and is only followed by a noun phrase. Keep in mind that لأن acts like إنّ and forces the noun right after it (الخسارة) into the accusative case.