You need several past tense columns, not just one. This is the tradeoff of any language with a rich conjugation system: there are many forms to master, but you can express subtle differences in meaning.
The page Laure cited is a good start to the two you need most often, the imparfait and the passé composé. Start by reading that, and if you want an answer from this site here’s one among many.
But besides those two, there are a few others that you’re bound to come across as you keep going in French. Some of them are:
- The passé simple : More or less the same meaning as the passé composé, but mainly literary.
- The plus-que-parfait : When you’re already talking about what happened in the past, and you want to say something else happened even earlier than that.
- The conditionnel passé : When you want to say you would have done something.
There’s a pretty comprehensive listing in WordReference’s conjugator, though unfortunately without those explanations. Pages like the one Laure cited are better for explaining them.
You might want to prepare columns for some of them, even if you’re not going to fill them out yet! One benefit is that when you come across a strange form in your reading or listening, you can put it in the right place.
So with fus and étais, you’re just looking at two different past tenses in this ecosystem. If translating into English, they would both be “I was”, but that’s because English doesn’t express the difference between them explicitly.
Fus is the passé simple. It’s normally used for one-time events. Expect to see it in books mainly.
Je fus étonné. “I was stunned.”
J’étais is the imparfait. It’s normally used for events that take place over a period of time.
J’étais bien content chez moi. “I was very happy at home.”
“Je fus” is called “passé simple”. It indicates something from the past which is short or punctually true or exists.
“J’étais” is “imparfait” (de l’indicatif) and is more common. It indicates something from the past with some duration or which can be still true (or not).
“Passé simple” is rarely used in common language, compared to “imparfait” or “passé composé” which is closer to “passé simple” in some aspects.
“Je fus surpris”. We’re talking about something past and punctual, at the moment we are talking about.
“J’étais médecin”. Still in the past but longer. This could always be true (“J’étais déjà médecin”) but the important point is I was for a while (or by my profession/state) at the moment we are talking about.
“J’étais surpris”. Can sound weird (in fact) but often used. It should say “I was ‘in surprise state’ from some time at the moment we are talking about”, but concretely, we rarely use this alone. For instance: “J’étais surpris qu’il dise ça” which relocates in time my surprise.
“J’ai été surpris”, “J’ai été médecin”. Can be long or short but is past (no more today).