Learn Dutch: Mijn opa had vroeger een grote verzameling. - My grandfather had a large collection in the past.

Had is the simple past tense of hebben (to have) for singular subjects like mijn opa (my grandfather). If the subject were plural, like 'my grandparents', you would use the plural form hadden.

Could I start this sentence with the word vroeger?

Yes! If you want to emphasize the time, you can say: Vroeger had mijn opa een grote verzameling. Just remember that moving vroeger to the front triggers inversion, meaning the verb (had) must swap places and come right before the subject (mijn opa).

Why is the word vroeger placed right after the verb in the original sentence?

In standard Dutch word order, adverbs of time like vroeger (in the past, or formerly) usually come immediately after the conjugated verb. Here, it naturally follows had.

Why is it grote with an added -e, even though it comes after een?

We only drop the -e ending on an adjective if it comes before a singular het-word paired with een (a) or no article. Because verzameling is a de-word (de verzameling), the adjective keeps its ending. Also notice that the double 'o' in the base word groot changes to a single 'o' in grote to keep the vowel sound long.