I am struggling deciding which tense to use if for example the first part of the sentence is one tense and the second part another, depending on time frame etc. Any hints or directions to other learning material please?
Can you give a more specific example?
If the sentence, in English, is "I was reading a book when the phone rang" -- you would need to use the imparfait and then the passé composé. Je lisais un livre quand le téléphone a sonné. Something was happening (imparfait) when something interrupted it (passé composé)
Or, you'll see two different tenses in a conditional sentence: https://www.lawlessfrench.com/grammar/si-clauses-conditionals/
Or, in a sentence with the subjonctif -- Il faut que j'aille chez moi