Show us what you've tried. Create a minimal example (a console program would be easier than full WinForms) that demonstrates the problem. Include your expected behaviour, the actual behaviour and the error messages. You might need to include the database schema too, depending on what you are doing.

I'm not finding any results on Google for ProbniDataContext. Is this your own type? What does ExecuteCommand() return?

As a side note you should be escaping input before adding it to a SQL query. Failure to do so will result in query syntax errors, unexpected behaviour or in some cases security issues.

Imagine if textBox1 containst the value "1", textBox3 contains the string "Mornin'" and textBox4 contains "Everybody".
What will we execute?

INSERT INTO ProbnaTablica VALUES (1,'Mornin'','Everybody')

You can see we have one quote too many.

Imagine if textBox1 containst the value "1", textBox3 contains the string "Hello" and textBox4 contains "World'); delete from ProbnaTablica; --"

What will we execute?

INSERT INTO ProbnaTablica VALUES (1,'Hello','World'); delete from ProbnaTablica; --')

Note the SQL comment at the end to get the rest of the statment fragment ignored.

This might not work, depending on the underlying implementation, but it might still cause an ugly exception that could easily be avoided.

Yes, both caused the exeption "Column name or number of supplied values doesn not match table definition",

Both examples I gave you, or both of your original queries?

As for ProbniDataContext, that is a type that was automaticly created when I added a .dblm file to my project...
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Ok. I haven't used C# to talk to a database, so I assumed this was a library.

ExecuteCommand appears to return an integer representing the number of rows modified. If you want data, you should be using the ExecuteQuery function instead.

It also looks like the answer to my aside is to use the argument list to Execute Query/Command:

int id = Convert.ToInt32(textBox1.Text);
PDC.ExecuteCommand("INSERT INTO ProbnaTablica VALUES ({0}, {1}, {2})", id, this.textBox3.Text, this.textBox4.Text);

// And

int inputId = Convert.ToInt32(textBox1.Text);
int outputId = PDC.ExecuteQuery(typeof(int), "SELECT ID FROM ProbnaTablica WHERE (ID = {0})", inputId)
textBox2.Text += Convert.ToString(outputId);

Expect some syntax errors in this, my C# is rusty.
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I managed to get the data inserting part of my program to work properly - I made a new table in my database where ID's IsIdentity property is set to Yes. For this part I only have to add exception handleing (the only restriction is that no data is null). Now I have to get the extraction part of this program to work - in the final app it should maintain a constant number of sentences in either a multiline textBox (this I am familiar with) or DataGridView (never worked with). The idea is to give the user insight into order and timeline of upcoming events - when event is triggered, it should remove it from the display area, but not from the database. I think this is very similar to the Tetris area that shows you which block is coming next - just this is done by extracting data from database and it's displaying text instead of pictures. If you have any suggestins or ideas on how to do this it would be great

You must design your schema to support this. So perhaps you should include an event date column. Then when you want to list upcoming events, you can select the events where this date column's value > NOW(), order by the date (soonest first) and limit to 5 events.

From my rough guess that the language is Serbian, it appears your table and column names are the equivalent of "test", so perhaps you haven't really looked into the schema part yet.