我吃了饭
I've eaten

我吃了一碗面条。我买了五个苹果。

了 goes straight after the verb and says that the event was completed — it got done, all of it. Chinese verbs themselves never change: 吃 is 吃 whether you ate yesterday or will eat tomorrow. The time comes from the time word (昨天, 刚才, 明天), and 了 only tells you the event is finished as a whole.

明天我吃了早饭就去你家。

Here is the proof that 了 is not a past tense. This sentence is about tomorrow, and 了 is still there: it marks 'once the eating is done', not 'it happened long ago'. The frame is verb + 了 … 就 …, meaning 'as soon as one thing is finished, the next follows'. 就 is simply 'then, right away'.

你吃了吗？——我吃了。我吃了一碗面条。

One habit to build now: give the object a number, or drop it. 我吃了一碗面条 is complete on its own, because 一碗 measures the eating. A bare object after 了 leans forward and wants a second clause — 我吃了饭就去. And in a short answer you simply leave the object out: 我吃了。

刚才我吃了一碗面条。昨天我买了三本书。明天我吃了早饭就去学校。

Say three sentences aloud. First, one thing you finished a moment ago with 刚才. Second, one thing you finished yesterday with 昨天 — and count the object: 三本书, 两杯茶. Third, and this is the important one, a sentence about tomorrow with 了 inside it: 明天我吃了早饭就去… If that third sentence still feels wrong, say it five more times. It is the whole point of this lesson.