Learn Hardcore Chinese: wǒ yǐqián bù dǒng zhè gè yìsi. - I didn’t understand this meaning before.

Chinese often marks time with time words instead of verb tense. 以前 (“before/earlier”) sets the time frame, so no 了 is needed. The sentence states a past state: you used to not understand.

To add a “now I do” contrast, say: 我以前不懂这个意思，现在懂了。

Why is 不 used instead of 没(有)?

不 negates a general state, habit, or ability (and is the default negator).
没(有) negates a completed event or experience. Here we’re describing a state/ability at an earlier time, so 不懂 is natural. If you want to say you failed to understand on a specific occasion, you’d use 没懂 (see next Q).

Can I say 我以前没懂这个意思? What’s the nuance?

It’s possible, but it shifts the meaning:

我以前不懂这个意思 = I used to not understand this (general state back then; often implies improvement now).
我以前没懂这个意思 = On previous occasions I didn’t manage to understand it (event-focused). For a single occasion: 那次我没懂你的意思。

Where does 以前 go? Can it go at the beginning or end?

Time words usually come after the subject or at the very start:

我以前不懂这个意思。
以前我不懂这个意思。 Putting it at the end (我不懂这个意思以前) is unnatural.

Why 这个 and not just 这? Is 个 required?

After 这/那/每, Chinese normally requires a measure word. 个 is the default, so 这个意思 is standard. Without a measure word, it sounds dialectal or very colloquial.

Can I drop 个 and say 这意思?

In casual colloquial speech (especially in the north), 这意思 is heard. In standard/written Mandarin, stick with 这个意思.

Pronunciation tips: any tone changes here?

Pinyin: wǒ yǐqián bù dǒng zhè ge yìsi.
Third-tone sandhi: 我 wǒ (3) before 以 yǐ (3) is pronounced like a rising tone: roughly “wó yǐqián”.
不 stays fourth tone here (bù dǒng), because it changes to bú only before another fourth tone.
这 is zhè (4), often colloquially zhèi before a measure word: zhèi ge.
个 is usually neutral tone here: ge.
意思 is yìsi; the second syllable is neutral tone.

Does 以前 imply that now I understand?

Often yes, pragmatically it hints at a contrast with the present. But it doesn’t guarantee it. Make it explicit if needed:

Now I do: 我以前不懂这个意思，现在懂了。
Still don’t: 我以前不懂这个意思，现在还是不懂。

What’s the difference between 懂, 明白, 理解, 知道, 了解 here?

懂: to grasp/understand (everyday, very common) — fits best with 意思.
明白: near-synonym of 懂; slightly clearer/explicit understanding. 我以前不明白这个意思。
理解: deeper, more reasoned understanding. 我以前不理解这个意思。
知道: to know (a fact); less natural with 意思 alone. Better with a clause: 我不知道这个词是什么意思。
了解: to be familiar with/understand in depth (about subjects/people), less about the meaning of a specific word/phrase.

Should there be 的, like 这个的意思 or 这个词的意思?

这个意思 = “this meaning” (referring directly to the meaning as a concept).
这个词的意思 = “the meaning of this word” (noun + 的 + noun pattern).
这个的意思 is usually wrong unless 这个 stands for a previously mentioned noun (rare in this exact sentence).

Can/should I use 了 here?

Not inside this clause. 我以前不懂这个意思 is fine. If you want to mark a change now, add another clause:

我以前不懂这个意思，现在懂了。 Saying 不懂了 means “no longer understand (now)”, which conflicts with 以前.

When would I use 听不懂 or 看不懂 instead?

Use a perceptual verb when you want to specify the channel:

Listening: 我以前听不懂他的中文。 (I couldn’t understand his Chinese by listening.)
Reading: 我以前看不懂这个句子。 For a meaning/concept itself, 不懂这个意思 is perfect.

Is 不太懂 more natural/polite?

Yes, 不太懂 softens the claim and often sounds more natural:

我以前不太懂这个意思。

Can I replace 以前 with 之前/曾经/过去/从前? Differences?

之前: “before (that point/event)”—often context-linked. 我之前不懂这个意思。
以前: broad “before/earlier” (neutral, very common).
曾经: “once/at one time,” often implying a contrast now. 我曾经不懂这个意思（现在懂了）。
过去/从前: “in the past/formerly,” a bit more formal/literary. 我过去不懂这个意思。

Why not use 是 (e.g., 我以前不是懂…)?

是 isn’t used to support verbs like 懂 in the way English uses “to be.” Use the adverbial negator directly: 不懂. Forms like 不是很懂 do exist colloquially (“not really knowledgeable about …”), but your sentence should be 不懂.