Learn Hardcore German: In der Vergangenheit hatten wir wahrscheinlich zu viele Zweifel, und das verhinderte den Fortschritt. - In the past we probably had too many doubts, and that prevented progress.

Because in used as a time-preposition (when?) always takes the dative in German. Here you’re locating the action in time (“in the past”), so you need the dative article der: in der Vergangenheit.

Why do we use the simple past (hatten, verhinderte) instead of the present perfect?

German has two main past tenses: the Präteritum (simple past) and the Perfekt (present perfect). In written or formal statements—especially in narratives—you often use Präteritum. That’s why you see hatten (not haben gehabt) and verhinderte (not hat verhindert). In colloquial spoken German you might say:
In der Vergangenheit haben wir wahrscheinlich zu viele Zweifel gehabt, und das hat den Fortschritt verhindert.

Why does wahrscheinlich appear after wir instead of at the end?

In a main clause the typical order is:

(Optional) pre-field, here In der Vergangenheit
Finite verb, here hatten
Subject, here wir
Adverbs of manner, probability, etc., here wahrscheinlich
Objects and other elements, here zu viele Zweifel
Putting wahrscheinlich in position 4 marks it as an adverb of probability.

What exactly does zu viele Zweifel mean, and how does zu work here?

zu in German can act as an intensifier before an adjective or quantifier to mean “too …” So zu viele = too many. Combined with Zweifel (doubts), the phrase means “too many doubts.”

Why is it written zu viele (two words) and not zuviele as one?

When you combine zu with a quantifier like viele, you keep them separate. Only when zu modifies an adjective directly (e.g. zu groß, “too big”) can it be seen as part of a fixed phrase. With quantifiers you write zu viele Zweifel, and if it ever needed declension it would still split: e.g. zu vielen Zweifeln (dative plural).

Why is there a comma before und in …, und das verhinderte den Fortschritt?

Standard German rules say you don’t need a comma before und when it simply joins two main clauses—but you’re allowed to insert one for clarity or to signal a stronger pause. In formal or handwritten texts some authors keep that comma; in everyday writing many omit it.

What does the pronoun das refer to in und das verhinderte den Fortschritt? Could I omit it?

The neuter demonstrative das here refers back to the entire preceding situation (“we had too many doubts”). German main clauses require an explicit subject, so you need das (or es, but das is more precise). You cannot simply say …, und verhinderte den Fortschritt, because then the verb would lack a subject.

Why is it den Fortschritt? What case and article is that?

Fortschritt is a masculine noun (der Fortschritt). It’s the direct object of verhindern (“to prevent something”), so it takes the accusative case. The masculine accusative article is den, hence den Fortschritt.

Could I use früher instead of in der Vergangenheit?

Yes. früher is a one-word adverb meaning “earlier” or “in the past” and is more colloquial. You could say:
Früher hatten wir wahrscheinlich zu viele Zweifel, und das verhinderte den Fortschritt.
The meaning stays the same, but the style feels a bit less formal.

Are there other ways to express “prevent progress” besides verhindern den Fortschritt?

Certainly. You can use:

hindern (often with an
- dative: an Fortschritt hindern)
blockieren (e.g. den Fortschritt blockieren)
aufhalten (e.g. den Fortschritt aufhalten)
Each verb carries a slightly different nuance—hindern is “to hinder,” blockieren “to block,” aufhalten “to hold up”—but they all can describe something stopping progress.