8. A computer program product for determining a combination of terms that represents subject matter of a natural language sentence, the computer program product comprising: one or more computer-readable storage devices and program instructions stored on the one or more storage devices, the program instructions comprising: program instructions to determine respective numbers of words from a beginning of the sentence to respective terms in the sentence that match terms in the combination of terms; program instructions to divide the sentence in a multiplicity of natural language phrases including a complex phrase and first and second simple phrases extracted from the complex phrase, the complex phrase being less than an entirety of the sentence; and program instructions to determine, based in part on (a) the respective numbers of words from the beginning of the sentence to respective terms in the sentence that match terms in the combination of terms, (b) whether all terms of the combination are contained in the first and/or second simple phrases, and (c) whether all terms of the combination are contained in the complex phrase but not contained in the first and/or second simple phrases, a confidence level indicating how well the combination of terms represents a condition or problem which is the subject matter of the sentence; program instructions to generate a table having a top row and other rows, the top row including entries that include respective words in the sentence that match terms in the combination, the other rows including entries that include the multiplicity of natural language phrases, the other rows including first and second rows, the first row including the first and second simple phrases, and the second row including the complex phrase; program instructions to determine respective numbers of rows from the words in the top row that match the terms in the combination to the first row if all terms in the combination are contained in the first and/or second simple phrases, or to the second row if all terms in the combination are contained in the complex phrase but not contained in the first and/or second simple phrases, wherein a determination of the confidence level indicating how well the combination of terms represents the condition or problem which is the subject matter of the sentence resulting from an execution of the program instructions to determine the confidence level indicating how well the combination of terms represents the condition or problem which is the subject matter of the sentence is further based in part on the numbers of rows from the words in the top row to the first or second row; program instructions to determine that the confidence level exceeds a threshold; program instructions to retrieve, in response to determining that the confidence level exceeds the threshold, contextual information from a knowledge base, the contextual information being related to the subject matter of the sentence; and program instructions to determine, based on the confidence level exceeding the threshold, that the contextual information retrieved from the knowledge base is a cause of the condition or problem which is the subject matter of the sentence.
8. A computer program product for determining a combination of terms that represents subject matter of a natural language sentence, the computer program product comprising: one or more computer-readable storage devices and program instructions stored on the one or more storage devices, the program instructions comprising: program instructions to determine respective numbers of words from a beginning of the sentence to respective terms in the sentence that match terms in the combination of terms; program instructions to divide the sentence in a multiplicity of natural language phrases including a complex phrase and first and second simple phrases extracted from the complex phrase, the complex phrase being less than an entirety of the sentence; and program instructions to determine, based in part on (a) the respective numbers of words from the beginning of the sentence to respective terms in the sentence that match terms in the combination of terms, (b) whether all terms of the combination are contained in the first and/or second simple phrases, and (c) whether all terms of the combination are contained in the complex phrase but not contained in the first and/or second simple phrases, a confidence level indicating how well the combination of terms represents a condition or problem which is the subject matter of the sentence; program instructions to generate a table having a top row and other rows, the top row including entries that include respective words in the sentence that match terms in the combination, the other rows including entries that include the multiplicity of natural language phrases, the other rows including first and second rows, the first row including the first and second simple phrases, and the second row including the complex phrase; program instructions to determine respective numbers of rows from the words in the top row that match the terms in the combination to the first row if all terms in the combination are contained in the first and/or second simple phrases, or to the second row if all terms in the combination are contained in the complex phrase but not contained in the first and/or second simple phrases, wherein a determination of the confidence level indicating how well the combination of terms represents the condition or problem which is the subject matter of the sentence resulting from an execution of the program instructions to determine the confidence level indicating how well the combination of terms represents the condition or problem which is the subject matter of the sentence is further based in part on the numbers of rows from the words in the top row to the first or second row; program instructions to determine that the confidence level exceeds a threshold; program instructions to retrieve, in response to determining that the confidence level exceeds the threshold, contextual information from a knowledge base, the contextual information being related to the subject matter of the sentence; and program instructions to determine, based on the confidence level exceeding the threshold, that the contextual information retrieved from the knowledge base is a cause of the condition or problem which is the subject matter of the sentence. 9. The computer program product of claim 8 , further comprising: program instructions, stored on at least one of the one or more storage devices, to determine whether a negation is included in a phrase in the sentence, the phrase including a term included in the terms of the sentence that match the terms in the combination of terms, wherein determining the confidence level indicating how well the combination of terms represents the condition or problem which is the subject matter of the sentence by the program instructions to determine the confidence level is based in part on the negation being included in the phrase of the sentence.