Used to introduce an opinion with confidence and conviction.
“I strongly believe the smaller apartment is the better choice for us right now.”
“I strongly believe investing in the better laptop will save us money in the long run.”
Task 5 is scored partly on how persuasively you argue for your choice, so opening a key point with "I strongly believe" immediately signals conviction to the examiner. Save it for your main argument rather than every sentence, so it keeps its persuasive weight instead of sounding repetitive.
Synonyms: I firmly believe, I'm convinced that, I truly think