Before = 之前

Ago, in the past, the last time = 以前,

“以前“的使用

我经常说错”before"。

语法：以前，+ Subject + Verb + Object

When we talked before: 之以前，我跟你说过... Your Chinese version means "Sometime ago, I had told you..."  The two English version has a different nuance.

The last time we discussed: 以前，我们聊天聊到过...

The last time, I lived in: 以前 我 住 在 上海...

语法：Subject + 以前 + Verb + Object
同样的意义

I didn't like ..... before: 我 以前不 喜欢... Here you used it correctly because "before" here means "ago", "somewhere in the past".

I didn't have ..... before: 我 以前 没有 ...
I often ... before: 我们 以前 经常 有....

不用“了”吗？ Yes, you don't need it.  However, you may have 过 to indicate past imperfect tense.

我以前沒有这样做过的 = "I HAD never done this way before."

If you are still confused about 之前 and 以前, try substituting "before" with "In the past" and see if the sentence give you the correct nuance.

“以前“的使用

我经常说错”before"。

语法：以前，+ Subject + Verb + Object

When we talked before: 以前，我跟你说过... (之前/以前，我[有]和你说过)
Last time we discussed: 以前，我们聊天聊到过... （上次，我们聊过...）
Last time, I lived in: 以前 我 住 在 上海...

语法：Subject + 以前 + Verb + Object
同样的意义

I didn't like ..... before: 我 以前不 喜欢...
I didn't have ..... before: 我 以前 没有 ... [过]
I often ... before: 我们 以前 经常....

不用“了”吗？ 不用