با کدوم فعل شروع کنیم ؟چرا؟

Based on the context provided, the correct answer is:

"I was going to tell you, but I forgot."

The phrase "I was going to tell you" indicates an intention or plan in the past that was not carried out, which fits the meaning of the sentence.

The other options do not fit as well:

"I will tell you" implies a future action, which doesn't match the context.

"I was telling you" suggests an ongoing action in the past, which is not what the sentence is conveying.

"I would tell you" implies a hypothetical or conditional situation, which is also not the meaning here.

So the best choice to complete the sentence is "was going to tell", as it expresses the past intention that was not fulfilled.