How Language Works (or my thoughts on such)

When you talk in your native language, are you actually actively thinking about what you want to say? Or do you just say it as it surfaces? Usually, when you're listening to another person in your native language, you just "know" what they're saying, right?

My question:

Does the human mind actually think about the grammar we use as we use it? As my Japanese has improved, I've started to notice a trend in my mistakes and those of my peers - which leads me to think that we don't actually use grammar as we're taught. I think that, in general, we have each separate meaning memorized.

For example, native speakers usually don't make mistakes with past or present tense when speaking - because it's obvious that it's either past or present. I don't confuse "saying" with "said" or "running" with "ran".  But when I, and my classmates, use Japanese, such mistakes are very common. For example, I might try to say 「猫にえさをあげました」 ( (I) gave food to a cat) but instead, I might say 「猫にえさをあげます」 ( (I) will give food to a cat). I think, in general, these mistakes come from learning present tense by default. We can recall the word fairly quickly, but not necessarily the tense we're looking for. With practice, such mistakes are much less frequent, but I find that they remain common in the newest words. This is particularly common among those who have been studying for the shortest amount of time.

I would like to further this by saying that mistakes in けいご・こたいご are also quite common. Japanese has several different modes of speech. I know a good bit of them and usually I am fairly good when writing, but when speaking, I default to the mode of speech that comes to mind first - casual speech. Think of it like slang and formal speech like writing an academic dissertation.

I'll have to think on this.