On one hand, the noun ところ means place and functions like other nouns. More importantly, it indicates 'time/aspect'. This usage is often listen in grammar books as ところだ.
- Tense: In English and Japanese we can change verbs to indicate tense (the time something will happen / happens / happened) - for example, will go / go and went = 行く and 行った。Tense tells us about the order and timing of events.
- Aspect: In English, we can also change verbs to indicate aspect (the state (ある), activities (走る), achievements (死ぬ), accomplishments (小説を書く), or semelfactives (ドアを叩く) of someone or something and relate to their completeness) - for example, adding 'be/have' and participles: I am about to eat, I am eating, I have (just) eaten / finished eating, and I have been eating. Aspect tells us that an event or situation has occurred, whether it was on one or multiple occasions, and whether it was part of a change or continuous.
Another factor is the type of verb.
- Tense clauses generally use event verbs such as 見る, 食べる, and 歩く, and
- Aspect clauses use state verbs such as ある, 出来るand いる.
So already we can see
- When talking about events such as 'when the dog saw the cat' or 'when I was out with a friend', we need to use Tense structures (verb changes + time-related words 今, 10時, 時, 頃, 前, 後, ったら、た途端, etc.).
- That is, rather than Aspect structures (verb changes + time-related words + aspect-related words もう, しゅんかん, 足ら最後、ことがある、and ところだ)。
Obviously its far more complex and interrelated than this quick summmary and includes other factors such as viewpoint, time of events, etc., but this brief summary helps explain this particular problem.
If your sentences feel unnatural, it's not the particles. It's more likely because you are using the Aspect conjunction ところ（だ） where it might be better to be using Tense conjunctions such as たら 'when', たとたんに 'the moment', or あいだ 'while':
Finally, when you do need to use ところだ, there are two ways:
1. As a sentence ending: Sentence + ところだ
- はるえは晩御飯を食べるところだ。Harue is just about to eat dinner.
- はるえは晩御飯を食べているところだ。Harue is in the midst of eating dinner
- はるえは晩御飯を食べたところだ。Harue has just eaten dinner.
- は売れは晩御飯を食べていたところだ。Harue has been eating dinner.
2. As a conjunction: Clause 1 + ところ + particle + Clause 2
- テリーと踊っているところをマーサに見られてしまった。Martha saw me when I was dancing with Terry.
- ご飯を食べ終わったところに真理子が訪ねてきた。Mariko came to see me when I had just finished eating dinner.
- 三章まで読んだところでねてしまった。I fell asleep when I had read up to the 3rd chapter.
を for the object Martha saw, に for the time I finished, and で for the place where I finished.