"andavai" don't exist, "eu andei " is passado\pretérito perfeito from verb andar (walk- simple past\ present perfect in english) . There are the conjugations "eu andarei" that is futuro do presente (simple future, future with will in english) and 'eu andava" is passado\ preterito imperfeito ( past with used to)

[3/3] To wrap this answer up, as a rule of thumb, 'andei' would be 'I walked' and 'andava' would be 'I used to walk' (for conditions 1 and 4) or 'I was walking' (for conditions 2 and 3; by the way, in this case you could also use a more literal translation and say 'eu ESTAVA andando', with 'estar' in the imperfect past plus the main verb in the infinitive). But it's not that straightforward, so be careful: a lot of times the simple past from English becomes the imperfect past in Portuguese and not the perfect one, especially with stative verbs (and of course 'to be' had to be one, to make your life easier, right?!). I know the difference can be subtle and tricky, but over time and practice you will eventually sort them out. Hope this helps!

[2/3] Sometimes the choice is a matter of what aspect one wants to emphasize with the sentence. Taking the examples from last paragraph, you can say either 'ontem, estava na casa dele' or 'ontem, estive na casa dele'. The difference? First one emphasizes that particular moment in the past when you were at his house (you place yourself in that past moment and lead your listener to do the same); I expect you're going to follow that up with another statement as to why that moment is relevant (maybe something happened right after that; 'ontem, estava na casa dele discutindo o projeto, quando, do nada, me senti mal e desmaiei'). Second one, in its turn, states a plain fact; it's over and you place yourself in the present. Maybe you're going to say something more, but that moment is no longer important ('ontem, estive na casa dele por duas horas e depois voltei para a minha própria casa'). With 'ficar', same thing (but taking into account the particular nature of the verb, as I said before): we might say, for instance, 'ano passado, FICAVA na casa dele todos os dias depois que eu saía da escola' (emphasis on that time; our minds go back to that period), but 'ano passado, FIQUEI na casa dele todos os dias e agora percebo o quanto isso me prejudicou' (you're in the present, stating a fact). Sometimes, yes, those tenses may seem to be interchangeable. 'Antes de te conhecer, eu fui/era um idiota' both seem fine, but still, there's a small, unconscious differente in perception (elaborated on in the previous paragraph). Natives will go with the most appropriate one without a second thought.

[1/3] (I had been writing this answer for some time, but you've closed the thread, so I'm going to post it here.) You mean 'andei' vs. 'andava', right? No, you can't use them interchangeably. We use the perfect past basically in the same way as the past simple in English: for completed, one-off actions and events ('Perdi o meu ônibus para o trabalho ontem e tive de ir a pé; andei três quilômetros e cheguei lá suando'). On the other hand, we use the imperfect past for actions that haven't come to an end in the past. That includes: 1) Old habits and customs ('Eu andava todo dia para trabalhar até que comprei o meu próprio carro'); 2) an ongoing event that was interrupted ('Eu andava de boa pela rua até que um cara me parou e anunciou um assalto' (probably every Brazilian has said this at least once in their lives lol)); 3) an event happening at the same time as another (be the latter perfect or imperfect — 'Eu andava tranquilo quando vi um acidente acontecer'; 'Eu andava tranquilo e sorridente enquanto ouvia Pink Floyd' (this one happens to me everyday)); 4) something being told as part of a story, report, or simple statement ('Eu andava muito devagar quando era criança'). Of course, those conditions might be simultaneous, most commonly a combination of condition 4 with one of the others. Understanding the idea behind the choice also depends on the nature of the verb in question. For example, 'eu estava na casa dele' refers to a specific, precise, exact moment in the past, since 'estar' (one of the translations for 'to be') usually implies a temporary state, so that sentence would probably refer to conditions 3 or 4 above. 'Ficar' ('to stay'), however, implies some state that lasts longer, so 'eu ficava na casa dele' will more likely be understood as conditions 1 or 4. But this is just an overall generalization; context is key.