Hi aginell4life,

First split the paragraph with space and loop through it.

Then, check the searched text index if found fetch the previous and next word with index value.

Finally, merge previos, searched and next text and insert in a list collection.

Refer below sample code.

HTML

<asp:Label ID="lblMessage" runat="server" />

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    string searchedText = "red";
    string message = "I have a red cap in my bag. The color of my car is red blended with blue Could you lend me your red shirt.";
    string[] words = message.Split(' ');
    List<string> sentences = new List<string>();
    for (int i = 0; i < words.Length; i++)
    {
        if (words[i].ToLower() == searchedText.ToLower())
        {
            string previous = string.Empty;
            string next = string.Empty;
            string previous1 = string.Empty;
            string next1 = string.Empty;
            if (i > 0)
            {
                previous = words[i - 1];
                previous1 = words[i - 2];
            }
            if (i < words.Length - 1)
            {
                next = words[i + 1];
            }
            if (i < words.Length - 2)
            {
                next1 = words[i + 2];
            }
            sentences.Add(string.Format("{0} {1} {2} {3} {4}", previous1, previous, searchedText, next, next1));
        }
    }
    lblMessage.Text = string.Join("<br />", sentences);
}

VB.Net

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    Dim searchedText As String = "red"
    Dim message As String = "I have a red cap in my bag. The color of my car is red blended with blue Could you lend me your red shirt."
    Dim words As String() = message.Split(" "c)
    Dim sentences As List(Of String) = New List(Of String)()
    For i As Integer = 0 To words.Length - 1
        If words(i).ToLower() = searchedText.ToLower() Then
            Dim previous As String = String.Empty
            Dim [next] As String = String.Empty
            Dim previous1 As String = String.Empty
            Dim next1 As String = String.Empty

            If i > 0 Then
                previous = words(i - 1)
                previous1 = words(i - 2)
            End If
            If i < words.Length - 1 Then
                [next] = words(i + 1)
            End If
            If i < words.Length - 2 Then
                next1 = words(i + 2)
            End If
            sentences.Add(String.Format("{0} {1} {2} {3} {4}", previous1, previous, searchedText, [next], next1))
        End If
    Next

    lblMessage.Text = String.Join("<br />", sentences)
End Sub

Screenshot