How do you differentiate:
I'd always longed to see... = I would always longed to see...
VS
I'd always longed to see.. = I had always longed to see..
How do you differentiate:
I'd always longed to see... = I would always longed to see...
VS
I'd always longed to see.. = I had always longed to see..
Both the auxiliary verbs would and had contract to 'd, usually with pronoun subjects: he'd, I'd, you'd, she'd, it'd, they'd, we'd. Whether spoken or written, you can't tell which one it is, if you only have the contraction.
But you can tell the difference if you notice what comes next.
Like all auxiliary verbs, each of these must be followed by a particular verb form.
In similar uses, the meaning is realized. But generally, it is known what use it is as a structure. For example, "Would" is usually used with the perfect infinitive (have + past participle), while "Had" is used with the past participle.
In your example, the sentence with had seems correct. Because it should be used as "would always v1".