The unfortunate news is that there is no concrete way of telling whether a phrasal verb is separable or inseparable. The best thing to do is learn them in small batches until you understand how to use them in various contexts. The great news is that most phrasal verbs are separable. However, when uncertain, it’s best to keep them unseparated unless the object of your sentence is a pronoun.
Here is why this is the safest tactic for using phrasal verbs when unsure: 1) Both separable and inseparable phrases will make sense in their unseparated forms when the objects are nouns or noun phrases. By keeping them unseparated, you are increasing your chances of being correct. While doing this, you need to observe how other speakers use the same phrasal verbs in their sentences. This will give you an idea as to whether or not you can separate them. 2) If the object is a pronoun, place it between the verb and its particles. By doing this, you are complying with the rule that requires for pronouns to be placed in the middle of separable phrasal verbs rather than after them.
Note: Some phrasal verbs can operate as separable and inseparable, and change in meaning according to how you use them. This is why it is important to understand each phrase and the different ways it can be used.
Example: Run into
a) Meet by chance: “I ran into Steven this morning.” [Inseparable – Means the same as “bump into”]
b) Collide with: “Apparently, Daniel ran his car into a tree this morning.” [Separable]
While it does make sense to say “His car ran into a tree this morning,” you cannot say “I ran Steven into this morning,” unless you’re being poetic. It is the same verb and particle combination, but the meanings and usages differ.
A willingness to experiment and make mistakes is essential when building your vocabulary. You need to remember that, unless you are taking an English proficiency test, you are under no pressure to be perfect. Use new words and phrases in your conversations, and remember to keep an open mind. If someone corrects a mistake that you make, be grateful for that, as they are showing you a way of how not to say something, therefore increasing your chances
of saying it correctly. Remember that one of the best ways of learning is from your own mistakes.