ともすれば is a formal expression used to indicate that something is apt to happen or tends to occur, usually with an undesirable outcome. It is often used in written language, such as essays, news articles, or formal speeches. The pattern suggests that given certain conditions, the action or state is likely to occur, sometimes without intention. For example, when talking about human nature, society, or abstract concepts, you might say 'People tend to...' or 'It is apt to become...'.

In terms of structure, ともすれば is placed after the plain form of a verb (e.g., 忘れがちになる, but note that the verb itself is in plain form) or after a noun with の (though less common, e.g., ともすればの失敗). However, the most common usage is with verbs in their dictionary form. The pattern is quite stiff and literary, so it is not used in casual conversation. Instead, in everyday speech, you might use ～がちだ or ～やすい to express similar ideas.

Because it is a set phrase, ともすれば does not conjugate. It often appears at the beginning of a sentence or after a topic to set up a cautionary statement. The tone is somewhat negative, implying that the tendency is a problem or a risk.