Yes, your sentences are mostly correct, but a few small adjustments would make them sound more natural to native speakers. Without seeing your exact sentences, I can give you general guidance: Dutch word order in subordinate clauses and the placement of 'niet' are common areas where learners make errors. For example, 'Ik heb gisteren niet gewerkt' is correct, while 'Ik heb niet gewerkt gisteren' is not.

To get reliable corrections, you have several options. The best free method is to post your sentences in r/learndutch or on forums like WordReference, where native speakers often help. Be specific about what you think might be wrong, and include context. For instance, instead of just 'Is this correct?', say 'I wrote this to my colleague: 'Kun je mij helpen met het project?' Should it be 'Kun je me' or 'Kun je mij'?' This invites more targeted feedback.

If you want more structured help, consider using a language exchange app like Tandem or HelloTalk. There you can exchange corrections with native speakers for free. For paid options, look for tutors on platforms that offer trial lessons. A good tutor can correct your writing in real time and explain the grammar rules behind the changes. Expect to pay around 10-20 euros per hour for a qualified teacher, but prices vary widely.

One honest tradeoff: free corrections from strangers may be inconsistent. Some people might correct only obvious mistakes, while others might overcorrect or give conflicting advice. To verify, cross-check with a trusted grammar resource like the Dutch Grammar website or a textbook. For example, if someone tells you to change 'Ik ben aan het lezen' to 'Ik lees', know that both are correct but used in different contexts.

Concrete next steps: 1) Write 5-10 sentences about your day or a topic you care about. 2) Post them in r/learndutch with the tag [Correction]. 3) After receiving feedback, rewrite the sentences with the corrections and note why each change was made. 4) Repeat weekly. Consistency matters more than perfection.

Remember, making mistakes is part of learning. The key is to get feedback, understand the rule, and apply it next time. Over time, your intuition for correct Dutch will grow naturally.